The Gaming Den

This question has a specific catalyst, but the answer should be broadly applicable to similar situations. We don't want to be John Wick, so what would a PC's superior (the master that taught them magic in a system where age = power) from their past do to mess with them without just flat-out walking up and killing them?

You want to figure out what the source of the problem is. Is the master just a jerk? Do they have some philosophical difference? Did the PC swap the master's longevity potion for dog piss as a prank before they learned how big a deal that was? A specific grievance is going to go a long way to suggest the kinds of actions that get taken. For instance, a philosophical disagreement could involve trolling with elaborate practical jokes proofs of the superiority of the master's position.

All else failing, a lot of the things a jerk wizard is going to do is just blocking what the PC wants to do. Opposing their motions at tribunal, poaching their vis sources, transforming their girlfriend into a pig, pissing in their oatmeal. You can probably get a fair amount of mileage out of just trying to make sure the master doesn't find out what the student's goals are to make it harder for him to figure out how to fuck with the student.

Even if the master is just in flat-out hate-mode, remember that it's Tormenting Master, and death is the end of suffering. Fatality is for the PCs friends, supporters, potential allies, etc; but really nasty curses are often better.

Demanding favors ("I raised you from a muggle so give me money/vis/a proxy vote", "screw this other guy because he insulted us and our lineage and teaching") is always popular.

You don't want him to be just a jerk, give him a reason why he expects something onerous and a plausible basis to ask from.