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.

I mean, there have been more than a dozen confirmed assassination attempts on Hitler (even suicide bomber attempts among them) and a few more unconfirmed possible attempts. The future did try their best (or just some brave people from the time itself).

Do you realise how many politicians get attacked every day? Do yourself a favour and check it out. It's a lot, it's bot not the safest job in the world, many get killed. There had to be an attack by a time traveler at least once

My favourite theory of time travel is that in the vast majority of timelines, travelling back in time guarantees that events will unfold as they always will, as given an infinite number of timelines in entropy there will be at least one timeline that forms a loop, and from that point effectively every timeline follows that loop. It may even be a multi-step loop, but it's still a loop and the time travel causes the events that leads to the time travel.

The future will always happen.

Time travel is possible. Changing the future however, isn't.

It has to do with wave function collapse. The cat is both alive and dead until its observed, at which point the wave function collapses to reveal which it is.

So let's say you go back in time and successfully kill Hitler. Well you're now observing that outcome from a tangent timeline. In our timeline, that wavefront has already collapsed. There's no takesies backsies. Its immutable.

If you can go back and try again, your just creating new wave funtion collapses, not changing the ones that already occurred.

Its like re-rolling a dice because you didn't like the outcome. You can re-roll as much as you want, it doesn't erase that first roll; its still there, imprinted on the quantum foam of the multi verse.