Well, if I go by a favorite Moffat miniseries, Jekyll, I’d say it’s clearly horror, with sci-fi trappings.
But just going by the original story, I still feel it’s horror. The issue isn’t the mechanism of the change, but more like the classic Chaney “The Wolf Man” quote:
“Even a man who is pure in heart and says his prayers by night may become a wolf when the wolfbane blooms and the moon is full and bright…”
Your goodness, even your godliness, do not protect you from evil. That’s horror IMHO.
Sure, there’s plenty of other elements, including hubris, the meddling in “God’s Domain” by trying to change human nature, all of which fall into more modern sci-fi- tropes, but I still think it’s horror at the base.
Back to the Hulk and why I like Maestro. The periods in the Marvel universe where Banner/The Hulk has been allowed to be happy is miniscule. OF course, because that’s not often an interesting story. And the various writers take a near(?)-sadistic glee in holding up some degree of happiness and violently ripping it away (Lucy and the football style) like in a major previous arc, Planet Hulk.
I couldn’t go through 1/1000 (feel free to add zeros) of Banner’s drama without my personality crumbling. And that’s one reason I love Maestro - given enough time (a given with Hulk’s toughness, healing, and arguably slow aging) , it seems inevitable that Maestro is the future.
Winking back to the OP though - I wouldn’t be surprised if Maestro wanders without pants, because he knew it would discomfort his enemies! And he was quite happy to have a harem in his original timeline…