Unicron wanted a body.
I was rewatching Transformers: Prime, as you do when you are feeling down. And instead of the intended two or three episodes I ended up watching through Predacon’s Rising. In the process something struck me.
When the show originally aired there was a lot of debate in the fandom over why Megatron was so dead set on Cyberforming Earth. Why he didn’t use the Omega Lock in the first place to revive Cybertron. Why he was going to waste the entire first batch of CNA and synthetic energon on Earth.
I realized that the answer was staring me in the face the entire time.
Cybertron was the body of Primus, capable of transforming into a mech form which he surrendered to become a planet and bear his children.
Likewise Unicron used to have a planetary form which he lost.
Now he wanted that back. Unicron still had control of Megatron. In the episodes leading up to the first time Megatron made the choice to Cyberform Earth he had done two very specific things. He had attached the arm of one of the origonal 13 Primes to himself and had spent time forging dark energon into a sword to wield it. Now the Primes were already corrupted by Unicron, the reason they fell, and the arm provided a direct conduit for the DE to flow into him.
Megatron was by that point simply a pawn of Unicron’s desires. Unicron put the idea to Cyberform Earth in Megatron’s processor and Megatron merely justified it as well as he could. Shockwave himself pointed out that it made no logical sense.