I have been on flights where unruly passengers have been deplaned prior to take-off...or at least threatened if their bad behavior continued.
If UA FA/GA/etc recognized and warned this guy of his "bad" behavior but still let him fly and now needed to divert, I would expect a response to my unhappiness from UA in the form of compensation for what could have (or should have) been an potentially avoidable IRROPS/diversion.