This is how Megatron's decision goes in Darkness Rising Part 3.
[Starscream stares at the now purple-eyed Megatron]
Starscream: Master. Are you alright?
Megatron: It's as if the blood of Unicron the Destroyer flows through my veins. As if I hear his very thoughts. I now know what I must do.
[Megatron grabs a shard of Dark Energon from the meteor]
Starscream: And what can I do to assist you, Master?
Megatron: Stop groveling and await my command.
[Megatron transforms and flies away]
Starscream: Soundwave, I fear that when our master reached for the stars he came down with a touch of space madness. Lord Megatron has not sound on his judgment since his return. Increase global surveillance.
Soundwave: (with Megatron's voice) Stop groveling and await my command.
Starscream: [annoyed] I'm not deaf. But if Optimus Prime lives, I believe it is in Lord Megatron's best interest that we ensure his enemies destruction.