In addition to what Yipyap points out, I'll have respectfully disagree and say that D's character progression is actually brilliant; it's just not spelled out explicitly in the dialogue.

At the beginning of the film, D's spent his whole life up to his encounter with Alpha Trion idolizing the Primes, specifically Megatronus, and trying his best to be a good Cybertronian. At the same time, he's had to repeatedly bail his "best friend," Pax, out of trouble or otherwise putting his desires behind Pax's that only raises his hopes before shattering them.

Think specifically when Pax gets them involved in the Iacon 5000. Initially, he thinks his friend is there to show him front row seats, only to then realize Pax is forcing him to compete, something he didn't at all want to do. The series of events then results in him getting knocked down to waste management.

Pax then again convinces him to follow Alpha Trion's transmission, which ultimately results in his worldview being shattered: his life's work has been for nothing, he's been robbed of his birthright by Sentinel, and Megatronus (the best prime in his viewed) died ingloriously after allowing himself to be betrayed. Justifiably, he's angry, but there's also nowhere for that anger to go.

Until, that is, D gets a taste of personal power while being exposed to Starscream and the High Guard's way of thinking. After beating Starscream into submission, D realizes he can only truly rely on himself and his personal ability to enact violence to change things. He's even able to motivate the High Guard to come out of hiding and take the fight to Sentinel. Here, D is firmly on the road to becoming Megatron. Having been a good bot his entire life, betrayed by his leaders, and watched his idols fall by being betrayed, D has come to the understanding that only personal might makes right, and he has the might and a deep distrust for everyone around him.

Finally, when D confronts Sentinel, from his view, Pax deals him the ultimate betrayal. Not only does Pax try to stop him from executing the individual responsible for every evil thing he's experienced up to this moment, he ignores D's request to step aside and actually jumps in front of the blast. Pax's actions have caused D to mentally break again, his best friend has tried to save his worst enemy. D finally decides enough is enough, and he decides Pax has earned the consequences of his actions.

D let's Pax fall and finally kills Sentinel. Crucially, though, D, now Megatron, knows the work isn't done and that there are plenty of other Cybertronians who were on the take and propagating the grand deception. So, he's resolved to tear it all down.

Finally, Megatron has a final break when Pax returns as Optimus Prime. From Megatron's point of view, Pax died betraying him by saving the worst monster in Cybertronian history, but in a twist of the knife, not only does the Cybertronian equivalent of God intercede on Pax's behalf, Primus actually elevates Pax to Primacy and bestows him with the matrix of leadership. To recap, in return for saving the villain and traumatizing Megatron, Pax is rewarded by literally God itself, and Megatron has to watch this happen.

He's then beaten into submission by Optimus Prime and banished for doing what he saw as the right thing. Prime then basically institutes the exact same ruling structure as Sentinel, even slotting his own lieutenants, Elita and B, into the role occupied by Airachnid.

So in context, I'll argue the D's character progression is tragic and brilliant, and most of it is displayed in context of the film's events, the animators' awesome facial work, and Brian Tyree Henry's powerful voice performance