Basically Starscream, because he is second-in-command and follows Megatron. But he is a major rebel and disobeys Megatron and tries to vanquish him so that he corrupted by his greed for power, to become the next leader of the Decepticons.

But Megatron is only insane because The Fallen through the AllSpark (Megatron's master) told him to start war between the between the two sides and that's why Megatron is convinced by the Fallen to rage war between two sides. The Fallen has also chosen Megatron to command them, but the real creator of the Decepticon insignia and army was actually The Fallen, once brother of the Primes.

See the difference?
Megatron was convinced by his master, while Starscream goes AGAINST his master.