You answered your own question: you got punished because you negatively retaliated against someone. Not only that but you also have a history of showing negative behavior in-game. As for why you were punished first it's because you were already close to crossing the line and this game was the one that pushed you over that line so-to-speak there. If that other player continues to act up then they will eventually be punished as well just like you were here. From Understanding the Tribunal:
4) That player in my game was far worse than me. Why am I banned???
a) I'll keep this one simple: retaliation isn't OK. Reacting to a toxic player by being toxic just incites that player to be even worse--it also ruins the game for up to 8 other players. It is far, far worse to watch two people yelling at each other all game than it is to watch 1 player ranting into blank space. Almost everyone in the game can find someone out there that behaves worse than they do--but so what? Finding someone worse doesn't justify your behavior or justify retaliation. We expect more out of our players. What players often don't realize is... those other players in your game that you thought were worse? Nearly 100% of the time, those players were banned even before you were. If someone is being toxic, just report them and move on. If you retaliate, both of you will be banned. It's really that simple.
Now if you want to actually discuss your punishment then please post your entire unedited Reform Card here for us to see (check your email/spam folder and the game client for it: the email contains one game while the game client can contain up to three).