All he says is "Thanks to my magic, the king will be implicated." I take it to mean he's magically lifting magical fingerprints or something like that. Of course, since a Cosmic Horror is involved, the Battlecasters will probably take over the crime scene like they did when Lynn's Brook exploded. I guess we don't know that they won't look at all the evidence and announce "Well, there's no way around it: It looks like we're the perps here!"....
Then again, "Thanks to my magic" is pretty vague. Maybe he means he'll just use his magic to mind control everyone into blaming the King.
EDIT: Also, this totally counts as him retroactively justifying his theft by defaming the Masters' characters, and I'm totally marking it on my Bingo sheet.
Come on, Chance! Throw yourself onto the tentacles! You know you want to!