Originally Posted by Dr Lee
Someone put it to me that he'd reached his limit... He'd seen his planet ripped apart through war... his friends all dying around him.
And then his long lost mentor turns around and brutally kills one of his few remaining friends and reveals himself to be in league with the Cons....
Prime had just had enough....
This. It's like what John Rambo said..."When you're pushed, killing's as easy as breathing." Optimus wanted to end this war once and for all, and why shouldn't he? Optimus killing Megatron was justified because it really was Megatron's plan all along. Thousands of innocent lives were lost because of Megatron throughout the three films. There was no reason for him to live. Sentinel just joined his cause to preserve Cybertron, even though he had no plans of becoming Megatron's inferior. Still, Optimus needed to destroy him as well, if only to avenge Ironhide.