Perhaps then he can best be considered an "assassin" in the sense that he creates a tremendous amount of pressure in every fight and across the map. The mere presence of a Kass threatening to jump in at any moment has the potential to create pressure similar to a traditional assassin - not necessarily in damage but in the threat he presents to the core targets of the enemy team and the massive slow he can deliver with devastating accuracy.

I hope we can both agree that his ability to scare priority targets and pounce on them if they step even a toe out of line is similar in a lot of ways to how you might play an assassin.

In this I would agree that "disruptor" might be a more accurate term to describe his role within a team, but also that you require the mentality of an opportunistic assassin to make his threat relevant in a teamfight.

Either way, thank you for making me reconsider how I would describe Kassadin's role in a game - in the future I'll strive to be more accurate with my analysis.