I think it's multiple things frankly. Spamton hints at knowing multiple higher concepts that Jevil doesn't even mention, particularly LOVE and DETERMINATION. These are concepts that are basically gone from DR lore all together, along with the facts of mister physiology and magic
Probably the best part of Deltarune so far; The secret bosses have their own vague, yet offputting secrets to tell.
What Jevil knows broke him, and now he's just a massively powerful chaotic mess who mindlessly enjoys the rampant pain and suffering of basically anything, including himself. But it makes him happy for some reason, and he doesn't seem to keen on sharing why.
Meanwhile, Spamtom is outright FURIOUS at the cards he's dealt with. The fact that he's specifically mad at Kris and these unknown players that seem to work behind the scenes suggests that whatever knowledge he picked up on, unlike Jevil, caused him to become a bloodthirsty, bitter psycho.
If we follow the trend, we could be exploring a "The truth drives you insane" type scenario with each of the secret bosses reveling in a different type of mania that really comes into play during their boss fights, and that's exciting because these secret bosses are ALWAYS the most engaging parts of the gameplay.
If the next secret boss has anything to do with the suspicious "Omega Flowey"-esque grins that people have been discovering all over chapter 2, I'm going to get extremely giddy. I seriously wonder what's coming next and it's a rare feeling I don't get from video games anymore.