Revenge was a not-insignificant part of it, yes. But separate from that was Khan's disgust for Starfleet's moral posturing, their claims of peace and freedom while they let their shadow division run rampant. It was, perhaps, foolish to feel betrayed by the future - but they had left Earth in the hopes that things would, one day, be better.

Instead, humanity had proved to be just as corrupt as they had in his time.

"Because the rest of your precious Federation deserves to see just how corrupt Starfleet really is." He shook his head, teeth bared. "Because Starfleet destroyed my entire species, and you would deny it as the actions of one man."

It had never been just Marcus.