The only purpose of life is to continue its existence, purpose of continue its existence is nothing, but nature of human as a living being.

Human have their right to do anything to continue their existence, but they have to pay the consequences on their actions. An individual have the right to kill another individual without any reason, but he/she must pay the consequences of being killed by personal vengeance or being executed by law.

Because human existence is meaningless, but human can only remove their existence by death, so if want to end up their existence, they will only end up with commit suicide. But there is human nature that human are afraid of death, or they are going be ceased to be exist. When human going to continue to exist, there are only 2 things to do to continue their existence:

1) Physically healthy, not living in poverty and disease.
2) Mentally healthy, not living as a pessimist.

When human existence is meaningless, why human going to find a meaning for it? Every meaning will be pessimistic... because human will die someday, a state will fall someday, the end of the world will come anyday, lover may die in the next day... everything is so unpredictable. To become optimistic, human must know everything will decay eventually, and human existence is only for the sake to be exist, then what human can do is be happy, and do not do anything regretful.

If you think serial killing is something not making you regret, just do it. But I think this is against the human nature. Human nature is to make human going to be exist in this world further, serial killing (mass homicide) is going to cease human going to be exist, this against the human nature. And... you must pay for the consequences on your actions.