Complex sentence: With Sideways killed and the threat of Unicron apparently gone, Megatron challenges Prime to a final battle, only for the energy released by their conflict to reactivate Unicron, forcing Megatron to sacrifice himself to end the cycle of hate that allows Unicron to exist.
Split into simpler sentences: With Sideways killed and the threat of Unicron apparently gone, Megatron challenges Prime to a final battle, only for the energy released by their conflict to reactivate Unicron. Upon realizing his mistake to continue the hatred, Megatron decides to sacrifice himself to end the cycle of hate that allows Unicron to exist.