I think ripping off faces is a little different from ripping off shirts. The former is more drastic and painful. But joking aside, i don't think whether Optimus was justified for killing the Fallen brutally or even for tearing his face off was ever truly what we took issue with. All our uncertainties about Optimus acting out of character derive primarily from his infamous line: "Give me your face!". Now if he had never uttered that spiteful, murderous imperative, I don't think many of us would have ever really questioned the manner of Optimus' execution of the Fallen. We would have just taken it as another instance of Optimus' bad-assery in battle.
Eh it's just a phase in Hollywood films. Back in the 80's action films you had to say some corny line that might end up as a catch phrase before you killed the main bad guy. These days you need something over the top violent with some slow motion involved if you can for that 300 type kill that is all the rage in action films. So with no one to really stop him with a script or anyone to say this is a bad idea Bay just sort of combined 80's action film ending with the modern cliche ending.
I think Bay just wanted to add something cool to Optimus' final battle scene..you know..Since he was missing for a large portion of the movie!!
