They could have an entire movie dedicated to Optimus Prime "falling" and "going bad", not that they would make such a movie, since that would alienate a lot of fans. It's something more suited for a series, with plenty of room for a proper chronicling of this "fall", and maybe even a redemption in the end. Of course, movie OP might be better-suited for going full Darth Vader given what he's done in the movies so far. If Ironhide had not crumbled into rust he would likely have made the replacement leader. It would still be a team effort to take down crazy Optimus Prime, most likely.
If Prime turned bad there is no one on the autobots as that could 1v1 him. They would all have to do what megatron did in the forest fight. My guess would the minimum of a 3v1 and atleast 2 of the autobots would die trying.
This is all that can really kill him I'd say....that or a stab from the back. It worked in ROTF, it could work again....could it?
Mudflap Prime would go to take his face, but seeing how ugly it is, would second guess that choice. That moment of indecision would lead to his downfall as Mudflap would instead destroy Prime's face saying "Nobody messes with me! In your face!"
