Hitler and his ilk are seemingly an unavoidable constant but that notion could only be because we’re based in a timeline where it happened and will never go away. Even if a time traveler did go back and stop Hitler itd probably create a branch reality similar to ours without Hitler, meaning we still live in the main unbranched timeline where Hitler certainly did exist. Even if this branch timeline without Hitler does exist we have no way of knowing whether or not a similar evil would take his place and possibly even commit worse acts of genocide. Also the branching of timelines itself could be catastrophic in ways unmeasured.