That seems to be a theme in this world of OPM.
When you desire something to the highest point, it feels like the world shapes that desire into reality.
Whether it is to become ultimately strong for the desire of becoming a hero (Saitama) or to hide the ugliness that he sees himself as (Sweet Mask).
He definitely did become a monster and that 100% explains his hatred for monsters. It also explains comments from random citizens in a few chapters about his constantly changing appearance. They gave hints to this a while back. Honestly I expected it from like chapter 11 or whatever though.
That’s a major plot twist considering Sweet Mask ABSOLUTELY LOATHS Monsters. I wanna know more on that though! Is it because he thinks he turned into a monster or something? : 0 Oh gosh,, if he really is a monster, ima be so flabbergasted.