This is 29 Acacia Road, and this is Eric, the schoolboy who leads an exciting double life. For when Eric eats a banana, an amazing transformation occurs.... Eric is Bananaman, ever alert for the call to action!
Regardless, literally everything I've ever seen (and a lot on tfwiki) attributes Megatron's slide into violence to Whirl beating him up while in custody. Maybe not like flicking a switch but Megatron's line "You made me. You taught me words were weightless without force" kinda spells it out.