I might just be on the outskirts due to my personality, but I can't say I've ever been toxic in a match. Sure, I've been annoyed and literally refused to chat for practically the entire game due to certain people, but I believe that every mistake should be held as if they were your own. Support doesn't play aggressively? Heck, you can change that by COMMUNICATING with them. Support keeps taking cs? Well, PLEASE DON'T TOUCH MY CS(And then it's K, like GP eating oranges). Jungler took the kills? Heck, I wouldn't even have this gold if he didn't come, and it's better to spread the wealth.
Onto the Twitch versus Vayne mentality on toxicity:
Vayne: These players tend to WANT to do the 1v5(Carry matches solo), and play this champ due to the reputation. Sadly, however, they realize that unless they're pretty much given a great early game head start, they'll suck until later on. This means that alot of people will rage when they don't get fed immediately.
Twitch: Though Twitch can stomp when ahead, he can be put behind and have little ability to recover. If you put a Twitch behind, then it's highly likely they'll start to complain, and will very often be the detriment of your team's morales and ability to play. On the other hand, they will also complain about any mistake you make that allowed them to die (Though it likely was due to Twitch player's own poor positioning).
Other ADCs: The majority of other ADCs tend to not have the same mentality to them because not many of them are large outplay champs(I'm excluding mentioning Draven), which means that their champs will be rather linear in their jobs. If these people mess up, it's much easier to pinpoint their errors, which means that it can be harder to blame it on somebody else. Sure, you can always blame another person for an action, but heck, some part will always be your fault and thus you will always be at part the cause of the action that ended in detriment for your team.