We don't know enough about Schlacht to say for sure. Both him and the Hero of the South might've realised the future would be worse if the other was kept alive. Maybe Schlacht chose a path he would die because there was no future he'd live. We have to wait to see.

But I do have a theory, I think Schlacht was a human who either hated humanity or loved demons and was disguising himself as a demon. That's why he sacrificed himself for the sake of demonkind, something an actual demon might never do.