Idk man... Not even Seth McFarlane can escape the primary rule of time travel. Don't change stuff, cause you don't foresee the consequences.
And I could imagine plenty of downsides to traveling back in time to kill Hitler.
Imagine coming back to the future and finding out, that because you killed Hitler, all the technological advances of the 40ies never happened, or happened much later, and the earth is at a 1960ies level of technology.
Or even worse: Because they killed Hitler, earth has been under a Stalinist type dictatorship for the past 400 years.