Again, this should be all interpreted in D-16 / Megatron's point of view.

[Millions in despair, I don't understand]

This phrase can be seen in two ways: Representing how he wants to destroy the Cybertron that Sentinel built, all that Sentinel ruled and owned, and Megatron doesn't understand why everyone got so desperate as he destroyed it all. In his vision, they should be grateful that he's destroying everything that someone as bad as Sentinel, the Betrayer, built and took care of. Megatron also doesn't understand how all Cybertronians, mostly the low-class who suffered the most, could follow Sentinel's system for so many years without noticing his antics and behavior.

[Putting your faith in the wrong guy]

Pretty self explanatory. Everyone on Cybertron trusted Sentinel "Prime", only to be betrayed by him. Once Megatron learned about the truth behind Sentinel, he got furious that he put so much faith in him.

[Talking about merit doesn't make sense]

This shows how Megatron would think that Orion Pax / Optimus Prime getting the Matrix of Leadership and speaking like he deserved it shouldn't make sense. D-16 wanted Orion by his side, he believed that Orion would join him to rule Cybertron, and then Optimus Prime appeared once he got the Matrix, leaving Megatron in disbelief.

[If a dark past creates an arid future]

This is about his view on Sentinel's situation in specific. How Sentinel killed all the Primes in the past, and how he's finally getting what he deserved. This would be his arid future, according to Megatron.

[I wish there was a tsunami over the cathedrals]

[The skies would be gained by dark smoke]

[It would also rain ashes]

The "cathedrals" here would be all those things that Sentinel once ruled, including the entirety of Cybertron. How Megatron wanted to destroy it all, to rule it all, to cause a revolution and be the one true leader. This is the beginning of everything, of a new era, a new revolutionary group called Decepticons.

[I wish there was a tsunami over the cathedrals]

[The weight of all the aces]

Now in this line, the cathedrals would be his relationship with Orion. A tsunami destroys everything, and D-16 / Megatron's actions destroyed absolutely everything they had, even if he never wanted it to end like that. Their bond being friendship or not, it's all over now. About the aces, ace cards either begin or end a match, depending on the game. In this case, Megatron is reflecting about the weight of ending everything he and Orion once had. The impact, the weight, the consequences, and how it would now change everyone's lives in the upcoming war.