There was one more that I can't believe I forgot to mention- Slade Wilson. I don't think any explanations are necessary.
Roy- I never meant to say he was a bad guy or that he enjoyed killing the cop, but once he found out that he did it, he WANTED to redeem himself. Even when he came back in Season 4, it became about him redeeming himself for having been forced to work for Noah. (Side note: I hope when Roy shows up, he tells Felicity that the reason he didn't make it to the reception was because if he had, he would have beaten up her father.)
Nyssa was an assassin for years. She was born into it. In her first appearance she kidnapped Dinah and Quentin Lance and threatened to kill them if Sara didn't come back with her, and then when Sara poisoned herself she tried to kill them anyway and was only stopped at the last second by Oliver. And while we haven't seen it, she has killed many people over the years both before and after she met Sara.
Malcolm... well, I was always torn on him. I loved the actor, but I loathed the character.
Vigilanted went on a killing spree last year, and the beginning of this year. you could say he was redeeming himself by going undercover.
I still think there is more to Black Siren than meets the eye. I agree that she enjoyed killing those people earlier in the season, but her killing Vince still looked like she really DIDN'T want to do it and DID feel bad about it. It may just be that she's suicidal and is trying for a death by vigilante.