ETA: re: "something worth fighting for"

Amusingly, that's probably more true in MoR than in canon, even if our Harry would never phrase it that way (or speak to Ron if he didn't need to). Here Tom Riddle's a bored sociopath without any strong connections; the only thing he really cares about is self-preservation, and that's more adequately assured if he doesn't pick fights with major wizarding governments or do crazy things like set up alternate versions of himself to spar with. In canon he's basically just Snake Wizard Hitler, and Hitler had if nothing else the courage of his convictions.