The thing I especially didn't like about Fixer is that he doesn't seem to care at all for Sev at the end of the game. He says "let's go" and doesn't seem to have any remorse. At least Scorch and Boss wanted to rescue him but Fixer didn't even wanted to help him.
They can, you know that. They were created to be better than droids because they can. And some of them leave at the end of Order 66, right?
The thing that keeps them following orders is that they are loyal to the Republic. It's what they're trained to do. Granted, though, the other clones are more favourable because they are more independent.
Not really but more than his brothers, that's for sure. If he received the order to kill one of his brothers, I think he would do it. He maybe would feel pain inside, but he would do it.
The way he abandonned Sev and the way he acts all along, both in the game and in the books, shows he's that type of man. In his little biography they even say he doesn't like names and prefers numbers!