He doesn’t do things that are supposed to happen, though. He makes sure not to screw up History as he knows it, except he always comes across a little thing that went wrong in his timeline and had a ripple effect of bad consequences, and he fixes them by doing good and creating a good ripple effect. It’s never suggested that any of those things are “supposed to happen,” just that they did happen in his timeline, and he finds better ways to fix them, without fucking up everything else, and creating a better timeline that is not his original one. The very fact that he risks fucking up big things show that nothing is “supposed to happen.” The opening narration mentions he strives to “put right what once went wrong”, so it’s strongly suggested that, if anything, he’s “meant” to do good, which is his own conclusion as the show goes on, somebody set him on this path to do good.

It’s never suggested that “putting right what once went wrong” means committing a crime that didn’t happen because without this crime, things were supposed to go worse. He fixes bad things by doing good. And, sure, we’re never shown that he needs to make sure a crime happens, but that goes against what’s suggested most of the time - imagine writing a show remotely hinting that maybe some war crimes that happened are justified or else it would have been worse. When he jumps into someone who’s about to do a bad thing, he just doesn’t do it and does good instead, the suggestion is that there’s no “'supposed to.”

The Kennedy episode heavily suggested that he was being influenced by some kind of psychosis from Oswald and he was going crazy himself, unable to stop himself from shooting, and that tracks with other episodes where he was sometimes overwhelmed by traits from his host, rather than having to make sure the crime does happen. He tried to save both Kennedys, and he failed JFK, but he saved Jackie. It’s not suggested that JFK “had” to die, it’s suggested that he “failed” half of it because the host’s influence was too strong.