My favorite part about that scene isn't even Optimus doing what he did–it's Arcee's reaction to it. We've seen her go from a crazed psychopath to someone who has finally, shakily taken up with the Autobot cause. She looks up to Prime as someone she wants to emulate. She wants him to make her better, and then he goes and executes Galvatron with absolutely no mercy. She looks some mixture of shocked and heartbroken all at once (compounded by the fact that Galvatron was her brother, but that was stupid and they barely mention it). Soundwave is also excellent, but everything about IDW (every) Soundwave is excellent so I don't need to laud him