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Evil =/= kill them all because they are good. Evil = remove them if they get in your way. It's probably not in your interest to kill them, as your own goals are furthered much more by their being alive. Ylu can convey the same evil feeling by doing stuff like always asking for money when a villager pleads with you to save their cousin from the undead hordes, whereas the Lawful Goodies would just do it. Or slitting the throats of prisoners because looking after them takes gold. Or interrogating them by removing body parts.

Gaining their trust is good. The easiest way to go about this is to help them in their quests. Pretend you are saving the cousin from the undead hordes because you're a good guy, just like them. Act as if you were lawful good. The difference between them and you is that they are doing it because they are nice guys, you're doing it to gain the trust of potential allies/mooks who will help you further your own goals of riches and power. Why waste resources on them till they become a problem? If they do come after you, point out all the other, worse guys still around, and shouldn't you all focus on killing them first. Helps you since it removes competitors and keeps you alive. Helps them because they are doing the most good with their limited resources, mostly limited by time. Priorities and all that.

If you desperately want to kill a character, Knockout followed by Bloodbath may well knock them unconcious for real, or almost there, and then you can drive them to negative bloodied with a Coup de Grace. Better if you go Dagger Master since then you can stack Critical Opportunity on that as welll, and if they are still up you can add Low Slash. If they are still up after all they, they have a ****-ton of HP, or you're not doing enough damage. If the former, well they probably don't do much else. If the latter, well, you still need them around.

Either way, both of those, although they will result in the death of one party member, and an angry player, probably will result in your characters swift death without a lot of planning. The rest of the parties first reaction will probbaly be to res him, and then he'll reveal all, and they'll slughter you, or you'll run away, and effectively not be part of the adventuring party any more, so still need to roll up a new character. Ditto if you're caught in the act obviously. So you're best bet after that is probbaly to stuff the body into a bag of holding and then at a later date burn it and dump it in the sea or some such where they can't find a way to res it. Then hope they don't cast a couple of rituals on the area to work out what happened.

Long story short - you could kill one character, but then you'd be screwed by the rest of the party one way or another, and have to roll up a new character as well.