I don't see much of a difference between killing players and NPCs. I don't kill players at all because I don't want to be killed by them. But if they had good PVP modes of play, like big naval battles (like playing x-wing or tie fighter), where you lost a navy ship instead of your own, then I'd probably like that. And I have 15 min in CQC and had no problem killing players there.
But what I won't do is kill innocent NPCs. They need to be wanted. And they have to have a big bounty. If it's only 200cr, then I let them go. Not because I'm credit hungry, but because I figure they just trespassed or something equally stupid to raise a bounty. In a res, I wait for them to attack innocent ships. If they interdict me, then that is there problem. But I wont take missions to kill traders or security vessels and etc. And I don't want to play as a criminal, pirating or killing cops to escape. It just doesn't fit my idea of a galaxy I want to live in. I want there to be law and order and I wish the game was more of a simulation of living in space and not an excuse to shoot things up.
I understand that power play, if I did it, could maybe allow me to kill "enemy" traders and cops if our systems were at war. But that's different than just indiscriminately shooting anything that moves.
Hell, I even wavered about killing clean skimmers, but I needed the credits after starting over.