Why are we not allowed to kill

[Ironside]

That has always been the worst and most irrelevant response. Who cares what other people want, realistically?

Caring about other people's perception of yourself is about one of the most essential things a human does, due to us being social beings. That's because it controls social contact, something that's essential to our well being and sometimes also our lives.

That makes sure that people care (due to empathy) or pretend to care.