Starscream was known for having been the one who previously lent his “loyalty“ to Megatron. Or more than loyalty, a false loyalty. Starscream attempted to eliminate Megatron in every possible way, but was never successful.

But, after ages, he finally achieved what he wanted: to become the new leader of the Decepticons.

And you were his guardian, no, his sparkmate. He himself had forced you to be his sparkmate, in exchange for not being chained in the ship's prisons, which you accepted, unfortunately. There you were now, next to the seeker's throne, now the new "leader."