I mostly play online, multiplayer, competitive games. Its my favorite type of game. Starcraft, Warcraft , DOTA, Heroes of the Storm, Chivalry, Overwatch, Splatoon, Counterstrike, PUBG, World of Warcraft, Rocket League etc.
I don't mind the toxicity too much in these games, but I know a lot of other people do. And it does get to me sometimes, where after a few hours of playing Overwatch I'll come out of it having a negative experience. I don't leave the game happy. And its not just losing, its when your team is bickering at each other. "Our healer sucked," "Nobody swapped to Solider" etc.
One interesting thing I noticed is that role/class based mixed with team-based objectives are the worst. Mostly because everyone wants to play the game how they want to play, and in a role/class based game with team based objectives -- everyone needs to be in sync. And in random groups, its less likely that's going to happen.
In arena shooters or PUBG I don't get that feeling as much. I don't need to worry too much about what other people are doing. I'm not telling some guy what gun he should be using. In PUBG, there's so many players that you're kind of a "winner" if you didn't die within the first minute. Rarely are you the first person in the game to die. Rarely do I come out of a PUBG game raging.
Heroes of the Storm and Overwatch are some of the worst I've seen. in HOTS, the objectives are so important. It's not like in DOTA where a carry can win the game if he jungles enough, everyone must work together in the objectives. If one guy just wants to do his own thing, there's a high chance you're going to lose.
TL;DR - After playing a lot of multiplayer/competitive games I've noticed the toxicity is most prevalent in role/class based games with team based objectives.