Roddenberry must've forgotten that he wrote "The Omega Glory", in which a corrupted Starfleet captain is a major character.
No, he had not forgotten. But he wanted traitorous Starfleeters to be a rarity
, not the norm - and the novels and TNG had also had quite a few that had made him antsy.
But, as pointed out, above, Gene had fairly little to do with the movies after TMP.
As "Creative Consultant", he was - contractually - given copies of every version of every movie script, and returned extensive memos on all of them. But no one had
to listen to him.