[bane]

If you subscribe to the multi-verse theory, then you can. Except when you "return" to your own time, you are forced to travel to a present where you did kill Hitler. Your own original time, where you didn't hill Hitler continues on as it was except you can no longer travel there.

Killer Hitler is like throwing a railroad switch and changing tracks, once entropy send you down the new track, there's no hopping back across. And even if you could you'd just be going to a present time where you failed to kill Hitler.