Romero has two states: Calm and Berserk. While in a calm state, he will walk slowly following the player, occasionally groaning and unleashing a roar. He will only attack players that are within melee range, and if he is damaged or hits a player while not within water, he will enter his Berserk state. While in a Berserk state, he will chase players down, constantly roaring and swinging his stage light, and he can buff nearby zombies by slamming down his stage light. Leading him into water will revert him to his Calm state.

Romero has 250K health in solo, with every added player adding another 250K health to that number for a total of 1M health in a four player game. If you do decide you want to try and kill him, the best way is just going to be unloading a bunch of ammo from Pack-a-Punched weapons into him until he eventually dies while he is in water to prevent him from ever entering his Berserk state.