I'm honestly fully convinced that these two are NOT the same person.
1.) Design: Not do these two looks NOTHING alike, but the original primes were described as being immensely enormous. So large, that they'd make normal-sized Cybertronians look like grasshoppers in comparison (think of Thundering from the TFP game: Thunderwing makes a deal with Megatron - TFP Game Cutscene ) Logistically, Megatronus would (and should) be a literal giant by Transformers standards. Yet, in RID, he's the exact same size as the regular bots.
2.) Characterization: In the Covenant of Primus, he's a troubled and tormented individual who struggles to control his passionate and aggressive impulses. He wants so badly to be a good person but can't help but feel insecure when standing next to his siblings (Prima especially). After being manipulated into accidentally killing Solus, he becomes so ashamed of what he'd done, he went into self-exile to atone and become a better person. But in RID, all of that characterization is virtually GONE, and he goes from a complex, tragic anti-hero to a straight up Saturday-morning cartoon bad guy whose motivations are about as generic as cardboard.
With this in mind, I'm thoroughly convinced that the dude in the right picture is an imposter. He's a mentally disturbed Cybteronian whose gone insane and thinks he's Megatronus, when he clearly isn't.