Hostility among players is an ongoing problem for many types of online games, where the competitive, “high stakes” nature of ranked competitiveness may foster anti-social behavior among game players. Not all games are created equal, however, as more online games now afford players with the opportunity to either play as characters with cooperative goals or choose less competitive game environments. Does the mere presence of a competitive environment or a cooperative teammate affect hostile responses in violent online games? An online field experiment was conducted to answer these questions using a 2 (game mode competitiveness: casual vs. ranked) x 2 (cooperative behavior: present vs. absent) x 2 (player allegiance: teammate vs. opponent) between-subjects design. Results suggest that teammates are typically more hostile to each other than to opponents, particularly in the absence of cooperative behavior. Previous work has postulated that opponents would be more likely to exhibit negative behaviors, but the results of this study indicate that players experience more negative behaviors from teammates, especially when the team is not working together. The theoretical implications of these findings are discussed.