Even if you get him to be the nicest person alive, someone else would become "Hitler" in a sense of no matter what you change the outcome stays the same.
Depends on the type of time travel you have.
1) Direct influence, where you directly change the future.
2) Fixed time, your actions become a part of the timeline keeping everything the same.
3) Multiverse, sort of a combination of the first two- you change the timeline yo go back to but your future still takes place in a parallel universe.
Well the situation in Germany was so bad and desperate and the dolchstosslegende was so widespread that another person would rise up if it wasn't for ol' Dolfie.
1) Direct influence, where you directly change the future.
2) Fixed time, your actions become a part of the timeline keeping everything the same.
3) Multiverse, sort of a combination of the first two- you change the timeline yo go back to but your future still takes place in a parallel universe.