Carmack is a great person, one of my coding idols.

An interesting piece of history is that it was confirmed in the biography Masters of Doom that he started iD software with Romero using company hardware while he was employed by Softdisk, and they had to come to an agreement to continue developing software for them (including Keen games) for a bit after they left in order to appease Softdisk into not suing them into oblivion.

Apparently, old habits die hard.