There’s the oft-quoted example of the butterfly effect, ie very small changes in a very complex system can have very big effects. You can criticise Hitler for many valid reasons, but one thing he wasn’t was “insignificant”; if he were, there’d be no desire to assassinate him. So even if you did succeed, given the impact he had on so many lives, you’d drastically alter the future/present, even if it panned out to be “better” without Hitler.

Say whoever replaced him was ineffectual and the war ended with reduced loss of life and destruction. In this timeline, maybe no German rocket scientists ended up in the US. The space programme loses some of its best minds, and happens more slowly (or not at all?) The space race resulted in a breath-taking amount of scientific advancement and spinoff technology, one strand of which eventually led to time travel. Now that you’ve changed things, time travel wasn’t invented in your lifetime, so either you vanish and the whole thing is undone, or your time machine does. So now you’re stranded in wartime Berlin. And you’ve just killed the beloved leader of one of the most powerful military machines in history.

Good luck with that.