People kill for many reasons, you can't just pick one.
Some kill to survive, some to defend others, some to impose their will on others, some as an act of defiance or hate or emotional distraught, some kill because they think it is right, some kill because they think it is an accetable solution to a problem, etc, etc.
Human have killed more to impose their will on others, and by that to increase their well being and chances of survival, than for any other reason probably.
I think people kill because they think it will make whatever issue they have, better.
Some kill because they simply enjoy how it feels.