So, it has come to this: someone displayed misbehavior in a LoL match in flagrant disregard for the game's rules, ToS, EULA, and Summoner's Code. That's unfortunate, and I'm sorry you had to deal with it. If this misbehavior happened in chat (including emotes or ping) and they don't respond positively to a single, courteous, constructive request to focus on the game, your best option is to mute that player. Submit a report after the match. This is equivalent to dealing with a noisy person in a library by asking them to keep it down and then notifying library staff and moving to another area.
Do not respond with misbehavior of your own! If you do that, your teammates may report you, and those reports would be valid. This is equivalent to dealing with a noisy person in a library by getting into a shouting match with them. Just because someone else started it doesn't justify you in continuing it. I'm sure you're familiar with the "s/he started it" trope, where two children get into an argument or fight and then try to claim innocence of any wrongdoing by claiming that they didn't instigate it. As any parent, teacher, or other supervisor of children will tell you, that excuse doesn't fly. When Dad is trying to drive you to Disneyland and your annoying brother starts making faces at you as he's previously been told not to, you should calmly ask him to stop, and then, if that doesn't work, calmly notify your parents of the problem. Making faces at him or shouting at him has never been a good way to get him to stop, and it makes you just as guilty of that as he.
Riot doesn't care who started it. Someone else's misbehavior does not justify your own.
From Riot's support knowledgebase:
- Simply speaking, retaliation is not an acceptable or justifiable behavior. An argument between two players can easily create a negative experience for the rest of the players in the game with you. Regardless of the other player’s actions, this does not justify your own behavior. You alone are responsible for your actions within the game.If you encounter a toxic player like this, the best option is to simply report their behavior and move on.
- Reports are a vital piece to the puzzle. If you are not sure of what sort of behavior is reportable take a look at the Reporting a Player FAQ
If you misbehave but a teammate or opponent does or says something even worse, you should definitely report them after the match, just as someone reported you for your own misbehavior. Yes, that's certainly possible. Reports are not a limited resource. Any time someone believes that another player violated LoL's behavioral standards, they can report that player. Punishments are similarly not limited: if more than one player in a game merits a punishment, they can both get a punishment. This can happen even if the players in question were antagonizing each other. The IFS doesn't need to weigh the severity of all reported players' actions and then "award" the "winner" with a punishment; it's not a contest.
Think of it like dealing with a noisy person in a library: ask them to keep it down, and then notify library staff and move to another area if that doesn't work. If you get into a shouting match with them, you're just as likely to be removed, even if you weren't shouting quite as loudly as they were. The goal is a quiet library.