Clyde Shelton - villain, hero or anti-hero?

I consider him mostly a villain. While his intentions seemed good, he started killing innocent people like his cellmate and Sarah who had zero to do with his tragic misfortune, and he killed everyone but the one man who he should have killed. He's a sociopath who felt he was justified in harming others and making Nick suffer because he didn't get the outcome he desired.

Definitely a villain. He may have believed he was doing the right thing, but his methodology was totally psychotic.

Somewhere in the middle, the night his family were murdered was what really broke him, and was hellbent on getting justice for their deaths, but he did not trust the flawed justice system and took matters into his own hands

I go for anti-hero.

Anti-Hero, though I rooted for him the entire film. He was clearly broken from his wife and daughter's deaths.