here's a good video on karma & rebirth explained by a respectable bikkhu.

before you start any thought experiments to prove/disprove, perhaps you can spend some time watching the dhamma talk video below to understand the concept in buddhist context a little better

yes. karmic action not necessarily only actions performed through speech or bodily act will guarantee karmic result/fruit (karma vipaka)

to be precise, karma is a volitional action. meaning it is an intention formed first in your mind, then leads to bodily action and verbal action.

and the fruition will ripen depending on whether certain conditions are met. a ripen karma is called karma vipaka.

it is predictable, but it's almost impossible for ordinary folks for you and i to predict the exact fruition of ones karma, unless you're fully enlighten 

yet if certain conditions are met with the karmic action performed, a guaranteed fruition may come.

if you're really interested to understand more, maybe you can start reading the actual words of the buddha's teachings on karma here:
MN 136: Maha Kammavibhanga Sutta - https://suttacentral.net/mn136/en/sujato

the thought experiment (container with ants) you created does not even have any relation at all in proving/disproving the phenomena.

It is rationalized when you choose which ants go into which container. The ants were chosen because of karma. To you it seems random, to the believers that's karma at work.

i'm afraid you might have a different perception of what 'karma' is. hinduism has a different perception on karma and so is jainism. the term 'karma' exists even way before the Buddha came to be. for the sake of discussion, i assume you're referring to 'karma' in buddhist context, so we can discuss coherently.

the term 'karma' in buddhist context is a 'volitional action', the intention or motive behind an action. it forms in our minds, all beings' minds, that precede any bodily or verbal actions. 'karma vipaka', the ripening or maturation of karma, is bound to happen when 'karma' is done. meaning the fruition of karma is guaranteed. the severity level of 'karma vipaka' depends on the severity of 'karma' done. it can ripen in the very present life or far future into the next life.

there are causes that drive us to have volitional actions (karma), I won't explain much of that because it is not relevant to your question but it is important to mention. to put it simply, the driving factors are the roots that manifest in 'karma' that can be categorized into unwholesome root (greed, hatred, delusion) and wholesome root (non-greed, non-hatred, non-delusion) they are in always dormant state, eager to jump out when conditions are met. buddhist monks' goal is to eradicate these unwholesome root, completely.
okay, now let us go back to your question at the very beginning: "Is karmic action over our next life guaranteed ?"

in my opinion and understanding, the result of our next life can be guaranteed only if certain conditions are met. and exactly what "conditions' as you ask further previously, that would depend on what sort of result you're expecting to achieve. with the right cause and nourished with the right conditions correspond to the expected result, the expected result would then be guaranteed.

if we refer back to your ant experiment. the CONDITION here for the early death of ants in container A is the administering of pesticide. the story doesn't end there. life cycle goes on in Samsara. the ants died may reborn into ants again or other beings in the next life, depends on the expiration & maturation of karma of the deeds of their past lives. 

Based on a thought experiment is also very tiny compared to the universe. At least I said it's likely.

But ignorant like you like to make a conclusion without evidence to prove karma is untrue.

The size is important unless you specifically mentioned Karma is untrue on earth.

For example, if you do your experiment to detect the existence of ghosts in a certain village that claimed to have ghosts roaming around. You did the experiment of 2 decades and finally, you concluded Ghost is untrue. This conclusion is not valid because the entire universe is too huge. Your sample size is too small to make sure a big conclusion.

This applies to almost anything including UFOs, Aliens, God, Deity or your favorite unicorn. 

Karma will relate to religion. It's hard to prove or explain to you. There is no certainty. This is not mathematics, 1+1=2. That's why certain people will eat vegetarian and no meats. It's sinful for them. I am sure you enjoy eating meats. 

You may donate some money to me......

Thanks in advanced.

Since the experts can't answer satisfactorily, let me answer for them. Ahem.....

Karma is carried throughout a "consciousness" existence where the next cycle is determined by that has been done in the previous cycle.

The one that most people understand is that if you have done good in this life, you will rewarded with a better life next cycle. If you did bad, you will be demoted in the next cycle.

What happens to the ants that got the free insecticide then? Well, first of all, they were very bad in the last cycle so they were demoted to become ants. Serves them right. That's karma!!! So how are they going to get the opportunity to ascend when everybody knows ants can't do good or bad deeds. What? Ants going on a homicidal rampage then get bad karma? Or ants did good deeds by helping poor ants so get good karma? They probably don'tt understand anything at that time. Only mindless insects programmed to do their work. Punishment enough I would say.

So if the ants died suddenly, the default is that they get to ascend up the ranks... a bit. Maybe become spider. After dying, the ascend further. Up to a point where their consciousness become self aware, and they know what is good or bad. At that point in the cycle, karma kicks in, and you'll get what you deserve in the next cycle based on what you did in the previous life.

How did I do? Ok?

You missed my part 2, as below:
That would easily explained by the fact their karma is carried forward together with their consciousness which is the only permanent thing in this universe.

Think of karma as a scoring system from 0 to 100. You do good, your score goes up. You do bad, your score goes down. So let's take your example of a rapist and a philanthropist in this life cycle. Both had a score of 80 in past lives. When these 2 were cyanided (haha), they goes into the next cycle but their score would be different. The rapist's score goes down so he gets demoted and become a cow. (If it was a serial killer, probably become any!) The philantropist's score goes up and he becomes a human Buddhist with higher chance to enter Nirvana. Of course, like the part 2 that I mentioned, you'll probably need to have a score of 100, a human Buddhist monk, to even have a remote change of entering nirvana. However, if you score nothing because you were an ant, your score will gradually go up, as you did nothing bad.

So each life cycle's karma is additive or subtractive and not all or none.

Haha, how? Better?

Will conclude my main reasons/points for this exercise soon.

It is predictable based on the scoring. An example of the scores:

0 - bacteria and virus
10 - microscopic organisms
20 - ants, insects
30 - animals
50 - humans, atheists
60 - humans, theists
70 - humans, normal Buddhists
80-90 - humans, devout Buddhists
90-100 - humans, buddhist monks

Say you are at 70, a normal Buddists. In this cycle, you are a thief. Not really very evil so minus 10 points for you. That means, in your next life, you are punished to become a human theists at 60 points. Haha. Say at that time, you died prematurely without any chance to earn any points. Your scores will be brought forward to next cycle, as karma follows consciousness which is the only permanent thing. However, not only is your score brought forward, it is added a bit because you didnt do anything bad. Instead of 60, you get +2 to become 62. That means that if you unfortunately died prematurely in the next few cycles, you may end up with a score of 70 again - a human Buddhist. This is to explain how lower tier life-forms, who doesnt know the meaning of good or bad, gets a chance to have a better life in the future. An ant will eventually get upgraded to the next tier eventually, and the next and the next, until it's consciousness assumes a form that is aware of good and bad. Once that happens, points will be additionally added or deducted based on karma.

And for those at top tier, the buddhist monk, it doesnt mean they get to enter Nirvana that cycle. It is not so easy. So each cycle, as long as they don't do bad, they will be at 100 and their chances increases each cycle. Who knows how many attempts at max 100 is needed before one can enter. Maybe 10, 20 or 100.

For you and me, we're at around 50 so there's still a lot of points to accumulate before we even have a chance to get the prize. we better get cracking earning more points!!!!

what if; the ants that were terminated were already set to be reborn into a better life? you were merely the tool to facilitate their next cycle? how would u be able to predict if you have no information on the ants that died on their karma status? Food for thought.

Ya ma. Already explained that their current cycle is not necessarily determined by past deeds. Instead, what they became was determined by pass deeds. Say a mass murderer human theist who became an ant because of karma. He already got his karma punishment so whatever happens after that is anybody's guess, whether cyanided or not.

Ya, that's what I'm saying.

On death, your karma points will determine what you become in your next cycle, and not what will happen in the next cycle. A mass murderer, on dying, gets his punishment by being an ant in the next cycle. He doesnt get to become human. That's his karma punishment. So as an ant in next cycle, whatever happens when he is an ant has nothing to do with the fact that he was a previous mass murderer. He already got his punishment by being an ant and in the new life, the ant is still being given the chance, like all life-forms, to ascend further and accumulate more points.

So whether you cyanide the ant or not, it doesnt matter. The outcome of this current ant life is not determine by previous life deeds. Meaning to say that at the time of death, the predictability is what the consciousness become in the next cycle, not the outcome of the next cycle.

Edit : to put it in another way, once you get punished or rewarded (to become whatever your score is), karma resets and all is forgotten. You start afresh with the points you have. You are not punished for endless cycles because of one bad life cycle. Every consciousness is given a new chance in every cycle.