Ah! Morty's canonically a seer/clairvoyant; would him having visions of the upcoming murders be okay?

He won't know that it's Caster who's the culprit, although if you'd like, he might pick up on one or, at most, two distinguishing features. He will pick up on the fact that some kid is totally going to get their ass murdered.

We can talk a little more at length about what is and isn't okay with you, if you'd like!