Bear with me, I don't quite know how to say what I'm trying to say.
I think Skittles has a lot of answers. I think the trauma and inability to cope coupled with isolation and fear of being held accountable simply for knowing or being a part of what happened is why he acts as jittery as he does. (Does that sentence even make sense?)
He is the one who explained about the different types of zombies. How does he know this? Yes it's possible he knows from observation and survival but does he really seem clear enough? He was simply being evasive so they wouldn't catch on that he knew more than he should. It seems like if they would have asked the right questions or paid a bit more attention they might have. I think Angel is dismissing him as being a bit touched. I hate to say this, but I think if Michael had been the one interacting with Skittles he woulda figured it out or trailed him for a while to pick his brain. I'm not knocking Angel at all it's just that his and Michaels brains work differently. If Michael does get the chance to meet Skittles or talks with Angel about him, they'll figure out he knows more and make an effort to seek him out. That's when we'll learn more about what happened to cause all this. Or at least point them in the direction of where to go to get answers.

I really hope this makes sense or at least gives you an idea of where I'm trying to go and why. Give me a little time to go back over the episodes I'm thinking of and gather my thoughts more coherently. Or for pete's sake if you get it and can clear it up go ahead.