On a philosophical level, no you can't do it. Justice cannot be preemptive, you can't punish someone for something before they do it.
That said, I do think we should drag quite a few politicians through the streets after they do horrible things. Serve justice appropriately as a deterrent.
You can get super stoner philosophizing with the idea that maybe histories monsters were necessary and killed off someone that would have been far worse. Perhaps Hitler killed some dude that was going to be far worse. Another path would be that by going back it would change the course of history and you prevent yourself from existing, kill Hitler and the baby boom generation winds up not happening and the USA doesn't become the dominant world power.
So uh, no, don't kill hitler/trump preemptively. Wait till after.