Starlin took a secondary superhero who had adventures like this:
And turned him into a cosmic hero battling an evil future self and grappling with the ramifications like this:
To the modern reader, it's overwritten and overwrought, but it was new and different and spoke to the times in a way that comics hadn't yet. This would all grow into the Thanos/Infinity War stories that swallowed the Marvel Universe and is now swallowing the Marvel movies. If you want more on the weird Warlock story and Starlin's writing, check out this great blog.
Also, I'd be remiss if I didn't take this opportunity to show off some of Starlin's incredible artwork that obviously helped inspire Rick Veitch for the flashback in Supreme #49. Enjoy!