This feels like a bit of a buggy interaction, Slayer killed an unrevealed card, and it didn't show which evil it was.
you had to click the card after it's killed to see what it was (unless you haven't killed the witch in that round yet)
Ah, it's there to prevent people from circumventing the witch's debuff. That makes sense.
I love how you can still interact with cards and even use their abilities when they are dead