Played a game with pantheon who would teleport into team fights

Well, ignoring anything else he may have done wrong (as you list nothing), it's possible that a pantheon would want to teleport in to a teamfight even though his ultimate is up: a) teamfight is too far away (pretty sure his ult isn't global) b) your team lacks sufficient CC to hold the enemy team in his ult-landing area resulting in the enemies vacating that area and his damage never landing anyways. Your team has no wards on the enemy side of the map. His thought is: "I can TP in or Ult in and either way get no damage off, but after if we win the fight I can't TP ahead of any stragglers trying to get away since we have no wards or minions in that direction, but I can still ULT ahead of them to cut them off."