I seem to be posting on here a lot. Anyways, I was reviewing some stuff for a Biology exam and came across this paragraph in my text book:
From Campbell Biology, Ninth Global Edition, Page 1121
"In 1848, a horrific workplace accident pointed to the role of the prefrontal cortex in temperment and decision making. Phineas Gage was working as the foreman of a railroad construction crew when an explosion drove a meter-long iron rod through his head. the rod, which was more than 3 cm in diameter at one end, entered his skull just below his left eye an dexited through the op of his head, damaging large portions of his frontal lobe. Astonishingly, Gage recovered. His personality, however, changed dramatically. He became emotionally detached, impatient, and erratic in his behavior."
Now, when I first read "emotionally detached," I thought "Prowl!" but with the "impatient and erratic" part tagged on, it sounds a lot more like Megatron.
So here's the bunny (or just take the idea that a processor/laser core/spark casing/whatever, when injured in a certain place, can remain active and functional, but the personality of the mech changes...):
Megatron was a loyal Lord High Protector. Until an accident while sparring injures that certain part of his processor. He spends a long time healing, but when he comes back to "work" or whatever, he's... different. Detached. Volatile. And Optimus knows he's hiding something.
And that's how/why Megatron became, well, the mech we all know and hate.