Referred to as being either:
1)Completely enraged to the point of beating up/freaking out at someone
2)So compelled and psychologically brainwashed that one begins to yell in German and conduct a mass genocide of innocent Jews, homosexuals, retards and gypsies.
Can also be enhanced/more specific by adding the suffix "on someone" or "on your ass"
Dude after Mike insulted John's mother, he started going Hitler on his ass.
After seeing his Dad's old communist and Neo-nazi friends, Greg began going Hitler on some of the kids during recess.