if he was level-synced he may be a veteran player that was doing a dutyroulette and wanted to make it a little less boring, i'm guilty of it once or twice, otherwise he was just selfish or isn't able to see the game from the tanks POV
fluid aura can atleast be used to finish mobs, avoiding the annoying knockback and still giving free dps on mobs hovering at 10%~ but beside that you are correct in your annoyance, maybe took it a bit far by leaving the instance though, it wasn't the end of the world was it?