I felt, at the end, in the scenes before he leaves for the last time, Ilmari could have thought a little more about his relationship to the world - as Kyllikki was saying, he made all the bad things happen just to end them again, but never entirely - and give Sampos father a scroll with a "spell", something like this writing: "If this scroll is torn, all of Ilmaris' fantasies will be unmade - reverting the world back to 10 years ago or whatever.", with the note to only do that after he left through the wardrobe, and that Ilmari doesn't know if it would work or if the father would really want to do such an extreme thing to the world, but the option might be there if he so chooses.
Because Ilmari essentially doomed the world to ruin by leaving and not defeating the demons he made, he could at least gift himself and sampos father a little bit of hope before fucking off.
He is responsible. And he knows that. He just figured that out in a hard way. But he basically says, "yeah, now it's your problem. k thx bye" and just floops out of there like he doesn't give a damn.