Unfettered, or Sociopath?

I've been working on a story idea for a while now, but recently an issue came up while describing the protagonist to someone: Their immediate reaction was that he sounded like a sociopath. Of course, it's a largely character driven plot, and half the point was to make the three main focuses as broken as possible - The Big Bad an immortal whose become so lonely and detached that he went mad and started causing chaos so that his god would send someone to finally kill him, the Mentor, an ancient seer whose become so reliant on his idea of 'destiny' that he refuses to act without first consulting his forsight, and The Hero, a kid who grew up believing he didn't have a destiny, in a community where everyones destiny is explicitly spelled out for them at birth, and fights the big bad not out of any sense of morality, or duty, but essentially because it's all he's got.

the problem lies in the fact that, because it's all he's got, he's determined to finish the job, whatever the cost, pointing out to a mook at one point, "I don't care about you, I care about him. Run, and you live. Get in my way, and you die."

He does have redeeming qualities - at least part of the reason that he fights the Big Bad is because he feels responsible for the destruction that's being caused, but there's a fine balance there, and I don't really know where I sit. If he's too good, he seems less broken and more simply put upon, but if he's too 'neutral' I'm worried about him coming off as unlikeable. thoughts?

I find nothing too wrong with him. Maybe you could add some more things to make him more relatable to others... even if it's probably only for a few people.

He can't be too good because "Fighting is what I'll do because it's all I can do" doesn't exactly sound like the ideal hero to me. As for being neutral, maybe you can make him a Deadpan Snarker? Maybe a Knight in Sour Armor or The Snark Knight. Maybe he also has this sense of superiority or whatnot and knows that if he doesn't go against the Big Bad, no one will. With that knowledge in mind, he knows that even if he's not completely good, he's not completely bad, either, and he can actually count as a Hero of some sort.

I'm sorry if that didn't help much, but that's what I would do if I was in your situation.

IMO unfettered and sociopath are hard to separate, unless you make the person in question regret or doubt his/her actions.

This really doesn't say sociopath to me, though. Seems like common sense.

So I'm thinking don't worry too much about neutralness -> unlikeability, unless you plan on making him an insufferable dick or cross a Moral Event Horizon.

Also, character development. Maybe he starts out doing this quest thing for whatever selfish, twisted reason but somewhere along the way he gets other reasons to finish the job, or (partly) resolves his issues.

That's a good point, about character development, since his reasoning tends to shift throughout the story. In the beginning, he does it because it's his 'destiny', but after talking to the villain he starts to do it because he realizes that The villain is merely causing havoc to inspire the hero to kill him, and so does it out of a sense that he's causing the problem, so he needs to stop it, and in the end regards it as more of a mercy kill than anything.