I like Sebastian and Cain. Cain is definitely a dark name. I usually see it spelled as Kane, but your spelling is good, too.
For some reason, though, when I saw that picture of Chris, I automatically thought of the name Gabriel. That's the name of an angel, but I think it'd be pretty cool to have this dark character, and then have him named after an archangel.
Edit: I'm exactly like you. I'll sit for hours trying to think of the perfect name for a character. Lmao.
I just kinda got hit with James Coleston, goes by Jamie.
I dunno man. That's just all I could think of as far as a name goes for him. But I totally get what you're saying about the name being really important. I'm the same exact way.