Oh, this thread... 
@ Fortlowe: FIrst off, refusing to call someone by the gender they see themselves as is rude. Would you insist on referring to a girl, who looks like a girl, lives as a girl, identifies herself as a girl, but happened to be born male a 'he?' Because biologically that is what they are? Sorry if I sound combative, and I realize Cole not being human is a factor here, but I've known too many transgender people who've had to deal with that sort of thing for your insistence on using "it" to not rub me the wrong way.
Anyway, my theory (and this is all theory based on reading the book, which in my opinion leaves a lot of things intentionally vague and open to interpretation) is that Cole might only think the murders sustain him. I'd initially forgotten that the murders don't begin until after Cole meet Rhys, but in a way it makes more sense. Before he meets Rhys, Cole just sort of drifts through the tower, watching and observing and accepting the fact that that's all he'll ever be able to do.
He meets Rhys and suddenly gets a taste of what it's like to actually feel substantial, to be noticed and remembered, to actually exist. To the point where, when Rhys goes away, now Cole knows what it means to want that, but he doesn't know to get it. The only people who acknowledge him are those who have lost hope, who he sees as potentially fading away just like the boy Cole did. Maybe it's a mercy killing, maybe it's the only thing in his power to do, maybe it occurs to him that if someone dies after seeing him they can't forget him. This is where the 'sustenance' thing comes in. I really didn't interpret his reaction to his victim's death as joy at all. There's no real reason for him not to, as I mentioned before, since he only has a fledgling sense of ethics, and he has no fear of being caught or punished. Again, it doesn't make it okay, but remember he's not a human being, and even the human who's memories he possesses was probably not the best example of a healthy, well-adjusted mind. Just a guess 
I think being acknowledged by those who call him a friends, by knowing he can make an impression on them by protecting them, he finds a different way to sustain himself. He learns he can be important, and make a difference. By the end of the book, he is no longer the invisible bystander he was before meeting Rhys, nor is he the desperate creature killing people because he doesn't know what else to do to feel like he's real. And now, in Inquisition, based on the write-ups about him and the fact that he's willing to join the Inquisition at all, I'd say he's moved well beyond that version of himself as well.