Why are we not allowed to kill

[Husar]

Humans are social animals. If we were 100% sociopaths(especially with low "social cognition") instead of just about that 1% of the population I doubt we would have moved much beyond the Primordial Soup.

Having empathy/conscience has a nasty tendency of limiting violent acts against fellow human beings. Of course it can be reduced by various factors but empathy seems to be working pretty well for a majority of humans. And that is IMO the main reason why most people find killing bad.


CBR

Yes, I was about to say (again) that feelings do play into this more than some seem to admit, it's not all about logic, for example who here chose his wife after calculating which woman would net him the biggest financial benefits?
Many humans do kill despite all the rational reasons etc. when their respect for their "victim" goes below a certain level, when they momentarily lose all empathy for that person. There are other reasons, like a hired killer may have more logical reasons to kill like monetary gain but then the percentage of professional killers is rather low I'd guess.

Soldiers also fit into this nicely as they are trained not to respect the enemies of their country and they seem to "make up" for this by having very strong bonds inside the army. I'll make a wild guess that when they start to realize their enemies are humans as well, is when many of them get nightmares and the like.