the killer, clearly mocking, left large, round mouse ears next to the corpse.
Yes, the clue is obvious, but many clues aren't very complex, and now the city is in complete chaos, which makes it even easier. Do you want to execute Swain? Sion will die at night, leaving 4 cities against 4 from the Abyss and Twi'st, who can be killed/blocked and won't be able to do anything? They'll use the second neutral development on Poppy.
Thorium, you thought I was Noxus, what do you think now?