Revenge of the Fallen Prime

From the end of that movie until he was remade as Galvatron, Megatron seemed clearly afraid of Optimus Prime... staying far from where humans or Autobots might be able to detect him, and even adopting a true disguise for the first time.

The Decepticon leader was in a weakened state, but had always regarded himself as the natural victory in any struggles the two would have.  He was certain he was physically more strong and skilled than Optimus Prime.  But he also believed his enemy was too constrained by wimpy compassion and petty moralities... too squeamish to do what was necessary.  That arrogant confidence must have disintegrated when he saw an upgraded Optimus finally hit his limit.

Megatron had a ringside seat, as the furious Autobot won a battle to the death against a murderous original Prime and then contemptuously mutilated its likeness as a coup-de-grace.