He simply had no fucks to give. He accepted whatever happened without question. It wasn't worth the effort for him to question things.

That was my take on his Strange behaviour.

He could be called many things. Camu intended that we see him how he is seen by society, as a stranger, but also be able to realise that each and every one of us could be a stranger on our own.

You can't act like a sociopath if you're not one. But I would guess Meursault is a psychopath, not a sociopath. While both are antisocial, there has been some talk of prosocial or maybe neutrally social psychopaths who still don't feel things but also don't go out of their way to hurt people. They generally just want to leave people alone and be left alone in turn. But as you see in the novel, that's really not an option in our very social world.

I haven't read it in a long time, but the impression I was left with is that Meursalt is existential in philosophy. But he takes it to a place of nothing matters so I'll just swim through life and what happens happens. (As opposed to a more positive form of existential philosophy - nothing matters and therefor I get to create my own meaning)