In my view, it would be naive to assume that killing Hitler before he became Chancellor would guarantee the prevention of the atrocities that followed, and it might even pose the risk of something worse happening. Events don't occur in a vacuum. It's similar to how dropping nukes on Hiroshima and Nagasaki were horrific events in themselves, but who's to say those events aren't the reason nukes have never been used in warfare since? Preventing those bombings might have saved the people in those cities, but it also might have significantly lowered the bar for using nukes, increasing the chance of a true nuclear holocaust during the Cold War, for example.

So maybe time travel is possible, and someone did try to change history by killing Hitler, but then realized the outcome was even worse. Now, they needed an additional assassin to deal with the first one.