A few things:
It's interesting to note that Sorin is not the character that has killed another party member. Reasoning aside, as "self-preservation" is also Sorin's motivation, which is apparently unacceptable.
Despite the laundry list of "very definitely evil" things Sorin has done so far, "killing an innocent" is not one of them. In fact the only creature he has directly killed has been the Zinogre, which I doubt anyone aside from Vash would raise much fuss about.
Sorin is not, as you keep saying, a serial killer. He has to kill to eat, and therefore to live. He does this out of nessecity, not pleasure, which you seem to be implying.
Lastly, the assertion that having White in his color identity should make him a better person is misconstrued. If you've read any of the supplementary material, it's very clear that Sorin is an unrepentant asshole that screws with people for his own amusement. He's Black and White. Both, at all times.