I've not seen the 3rd movie but you have expressed very clearly my issue with Bayformers Prime. He was one of very, very few Transformers to get any characterisation in the films and then it was just wrong.
Optimus is supposed to be a hero, to stand for something better and be an inspiration. Bays version wasn't the Optimus I grew up with, and to my mind he certainly wasn't a hero.
The way he executed an unarmed, restrained Decepticon prisoner and the whole "give me your face" line in ROTF were just
From what you have written, it sounds like he was much the same in the 3rd movie.
Can you imagine Captain America shooting a no name Nazi/Hydra soldier in the head while he was being restrained by American soldiers in the new film? No way. He is a hero and thats not what heros do !!
The new Batman films might have been "dark" but the second film is practically all about "the line he wouldn't cross" even when it would have been easier to do it.
Bob