That assumes you’re only pulling off one time-travel assassination. What if you just keep doing them until the lack of the relevant people shifts the political climate by itself? Like in the "go back and kill Hitler (or just push him onto a different life path at a young age, if you have time travel, you can probably remove someone as a leader without just killing them), sure, maybe a different fascist rises in Germany instead, but if you take out that one too, and the next one, and the next, eventually someone who isn’t a fascist will manage to get into office, or if you keep it up long enough, the german fascist movement will run out of viable leaders, and cease to be a major factor in the political climate of the time.