You can be competitive without being a dick.There are different game modes,
Quick Play - You play for fun. You can pick what ever class and just practise or goof around. Sure you can try to win but you should never get upset in this game mode if your team is underperforming.
Competitive - A SERIOUS game mode. Here, there is ONE objective and one objective only. To WIN. No matter what. Fun or not. Here, you are 100% dedicated to the game, and take it serious, like it's your job or anything else you also value very high in your real life.
If you don't care to win or want to have fun, for the love of god, stay out of competitive. It's not meant for fun, it's meant to push ranks and win.