Not going to lie, I am not surprised Sweet Mask is a monster after all, what with the face changes during fights and all the beatings he takes while barely reacting to them.
Also, the way he tells his story is the typical way all monsters explain their backstory...
Actually, are we sure Saitama isn't a monster? The story of how he got his powers is also similar to how monsters usually develop, but I guess something happened and he ended up as he is now? Maybe in the past he was Blast, but something happened and he monsterized into Saitama after some event, or some weird thing like that.