Toxicity in games exist and will never die. If you play online multiplayer games you will run into a toxic gamer pretty fast. A toxic gamer will never admit that it's their fault, they will always blame their teammate, or they will be a sore winner, trash-talking constantly even though they are winning. In my experience, I notice a lot of toxic players in Rocket League and Call of Duty. Rocket League toxicity comes with the quick chat options you have, for example if you get scored on the toxic opponent will "What a save!" spam the chat until they can't, this can also occur if you have a toxic teammate and you get scored on. Even though the goal could be off of their mistake they will "What a save!" you every time.
I've dealt with a lot of toxic people and honestly there's no way around it, people are a$$holes and you can't make them nice so here are a few ways I've dealt with it.
1. Never give in
If someone is being toxic don't give in and retaliate with your own form of toxicity that's what they want, instead stay quiet and let you playing do the talking. In the end especially if you win, it will be much more satisfying.
2. What you don't see can't hurt you
For most games, there are ways to turn off in-game chatting abilities or muting people. If you can't see the toxicity it can't affect you.
3. Motivation
If your own teammate happens to be the toxic one, try some motivating and encouraging words. Sometimes people are just not having a good day and want to take it out on others, so if you try being nice to them there's a slight chance you are able to change their mood and turn the game around.