Humanity can have so many meaning, depends on what is being used for. Human nature or human species as a whole.
Unless you agree that being killed is suffering, horror, fear, by putting yourself in their shoe, then you will agree that any action that create suffering, horror and fear to another human (or yourself) is a sin. However if you think that being killed is relief, detachment, end of worldly suffering by putting yourself in their shoe, well, you will just step on it like a cockroaches. Well, in emergency or reaction for self defense, your mind will set priority to self survival, relieving of self suffering and fear, thats why you dont feel the sin at that moment.
The Realization of "sin" also depends on how you are mentally and emotionally attached to a particular subject, either by realizing the outcome of action or pre-bounded by rules of religion, politics and understanding. Without attachment, there would not be realization, and hence consciousness cannot be achieved.
Talking of is killing is a sin, you must first realize that there is a fear of sin, then you must agree that fear is an outcome of sin, then you must react by reducing fear by reducing sin, and if the killing created fear for you, you will stop the fear with not killing, and hence you are not sinned. Well this is explanation by fear. As fear is the summation of all consciousness, attachments, understanding and rational thinking. Without fear, well, anything is possible. And what makes a gladiator a human killing machine, is they knows no fear.
If explanation by its ultimate purpose of life, sin will be in hierarchy. Your conscious mind will tell you that killing an ant that infest your sugar is sin less than killing a human that infest you backyard. Well, rather saying killing will cause more pain for a more intelligent species than a less intelligent species, (pain is to the nerve innervation, and it is relative, and stimulating one nerve of a one nerve organism is equally as stimulating thousand nerve of a thousand nerve organism), you would say that pyramid of food is the source of why organisms above kills organisms of the level below.
If explain in terms of moral judgment, sin becomes rather relative. Moral origins from self. If you think that you can strongly support your reason for an action done, it had not been sinned.
No matter SIN is stated by laws of religion , or SIN is generated by fear , or SIN is relatively realized by moral judgment, "SIN" is still "the wrong doings".
And only if you believe in that you have SIN , than you have the faith.
Well, as i write this reply, My awareness tells me that i had think of this issue for one hour, hence i have SIN to my parents , since next week is my final exam and i should study hard as agreed to my parents , no more spending useless time on internet. Well ...