There is, an albeit loose, and probably accidental, arc in Bay Prime's character. First movie he is relatively sane, but the betrayal of his brother (megatron) has him all sad and ashamed, to the point where he's cool with making the ultimate sacrifice by having the allspark rammed in his chest.
He doesn't do that, and megs dies. And he still skewers Bonecrusher on the highway. Which, violent but it is war.
Anyways, in revenge of the fallen, he dies, and this is where his aggression really elevates. I mean, the man wears a corpse and starts taking faces. Optimus definitely did not enjoy dying.
Dark of the Moon, he is betrayed both by the humans he has decided to protect and his mentor. To say this made him jaded and bitter would be an understatement. Sure, the Autobots needed time to regroup, but he essentially lets the Deceptions invade unimpeded to prove a point to humans they shouldn't have exiled his crew, then takes care of megatron and sentinel himself, while losing an arm in the process.
In age of extinction, the humans betray prime further as Cemetery Wind sets out to exterminate them. By this point, optimus has lost more friends and is blasting evil humans and wondering what any of this all means.
And by the last knight, even though you have Unicron mucking with him, he straight up becomes nemesis prime. Which, I had always wanted the more psycho he got. Like, if you're gonna do it, do it.
Anyways, where was I going? I guess it makes *sense* over the course of five movies for *this* Prime to be a maniac. Here, in Rise of the Beasts, I felt he hadnt had all that jive happen to him yet and he's already THERE.
Granted, they could expand on this in the next one like you said. Time will tell.
(Also, having Scourge BE future Optimus (or A future Optimus) still would have been a logical step for him, but whatever. Scourge is Scourge, whoever that is)