Optimus does what needs to be done, but I agree that his dialogue made him seem more brutal than he was. He isn't the kind of guy that says "Kill them all." Or rips someone's face off unnecessarily.
When he says "Megatron has to be stopped, no matter the cost.", to me it implies that Prime is ready to sacrifice himself, in order to protect everyone. But "Kill them all.", that's just too bloodthirsty.
I would have loved if they portrayed him as someone who doesn't like conflict, but can open a can of whoopass on you if he has to.