But along those lines: was Khan the *worst* thing he warned them about? Doomsday Machine? Whale probe? Borg? etc...
Who's to say he didn't warn Starfleet about some of those things as well? However, compared to indestructible planet killers and genocidal techno-zombies, Khan probably presented a unique opportunity, and Starfleet probably believed
(erroneously, it seems) that they could control him.