I'm thinking of potentially destroying some guy/girl's marriage over an online game
Do you really hate this person?
No, I'm just I wish them well in their relationship.
A game is not reality, people play for fun and to escape. It is very wrong to ruin some one else's relationship over a trivial game.
I guess I'll just tell him why I'm a little bit mad at him.
It's good to share your feelings with others. That's a much better solution than trying to ruin their relationship.