I'm having a bit of a delimma deciding what alignment a character belonging to one of my players should have.
Thorn has a hunger for blood. She realizes that this cannot be rightly sated on the innocent. She more or less ignores petty evil, mostly targeting a grander scale of evil; she'll ignore the starving commoner who steals bread from the local foodshop, but if the owner of that foodshop came and tool that commoner as slave labor in retaliation, Thorn would be all over that.
Her methods of dealing with the evil people in question (crime bosses, tyrannical nobility, cult leaders, jailers who abuse wrongly accused prisoners, etc.), are rather brutal and malicious. Skinning them alive, for example. Her actions against these people nearly always result in relief and security for the innocent population, or at least a sense that someone is looking out for them.
Rather than a paladin, who may take an evil person and either try to redeem them or do away with the evil quickly, She on the other hand realizes that often any repentance is false; and that a quick death is an easy copout. She believes in truly making them regret destroying the lives of others, and if it means she gets to torture them in the process, then all the better to feed her thirst for blood.
Knowing that she does her evil acts only to evil people for the greater good of the world, I'm inclined to suggest her alignment should be Lawful, Chaotic, or True Neutral. I doubt someone with an Evil alignment would genuinely work towards the greater good, even if she sees it as a partial justification for her bloodthirst.