Transformer characters come and go, to the point that the film series' only true constants are Optimus Prime and voiceless Autobot underling, Bumblebee. For the latter, Dark of the Moon is largely business as usual, but the same can't exactly be said for Optimus. The ending of Transformers: Dark of the Moon isn't an easy one for the Autobot leader or, for that matter, the viewers who have cherished memories of the leader as a kindly figure. Thoroughly fed up with the predictably treacherous Megatron (Hugo Weaving) and the rather more surprisingly unreliable Sentinel Prime, Optimus opts for extreme measures and brutally kills both villains. This is especially dark considering that Megatron saved the Autobot's life in the preceding battle and was actively negotiating an armistice between Autobots and Decepticons — while Sentinel Prime was utterly defeated and helpless by the time Optimus was done.
The movie arguably comments on Optimus' actions with stunned looks from key characters, and while the mood soon shifts to flag-waving happy ending mode, the heavily damaged robot is visibly burdened by these deeds. As such, Optimus' drastic decision acts as a neat nod at the franchise's upcoming, darker direction. After all, the Transformers' hunted fugitive status in Transformers: Age of Extinction and Optimus' own stint as Nemesis Prime in Transformers: The Last Knight are far easier to take in when you remember the grim foundations this movie subtly builds.
