For example, if Allied Forces had managed to capture Hitler, would they execute him on the spot or arrest him so that he can stand trial for war crimes and crimes against humanity?
Back then, i'd say 99% chance they would of placed a bullet in his head, and i would do the same. Waste of time and money putting him on trial, just like 99% of todays criminals.

The movies are getting over-analysed and compared to old-school TF. 90% of the audience probably haven't seen G1 so they don't care if Prime doesn't have a heart. Kids these days want tough good-guys, not by-the-book heroes. We don't what prime went through in-between movies. Maybe he was tortured by Demolisher so this was payback, or maybe there was a bounty out on his head for his death. All this shows us is that Prime is serious and wants the Decepticons gone or dead. Yes he's changed since the first movie but that's life. Move along.