A scientific study on the subject.
CANNOT POST LINKS. 
Look on Google scholar or google: Toxic Behavior in Online Games - Jultika
Here is a definition from Urban Dictionary on the social meaning of toxic:
Adjective used to describe usually very negative person, that bch about everything, spread unnecessary hate or just talk s* about others. You can meet these people in any online game comunity and they are the main coin of online gaming.
"Yo mike is so toxic when he is loosing. "
“Why you gotta be so toxic Tyler ? please calm down.”
Or:
Toxic is a group of people who are rude and can’t be nice. They are not true to people around them. They need an attitude check. Their personalities are so unappealing its make the people around them suffer and turn rude as well.
Discussion:
What is considered toxic in overwatch in particular? Asking a guy to swap is a bit toxic but in chemistry toxicity become problematic in a specific dosage. So asking one time is fine asking multiple times until a player is forced to pick somthing he doesn’t consider good in competitive, means being toxic. It is an harrassement.
People should consider their picks in relation to all the variants related to the game and to his skill. Even not feeling to swap a role and being forced to it from a team member is toxic.
Instead trying to work together in a hard to coordinate and have a synergy team could lead to unexpected new way to play the game.
Toxicity is defined around a social interaction. There is no toxic behaviour in the same game if you play with bots but it become toxic a person who trust his opinion on what you should pick more then your opinion on your personal capability. Tho, asking for a change is good for the game in little dosage. People could get offended for that after a persistent demand they cannot provide.
In the end if you really need a 222 comp or a specific meta, a team that is able to flex in needs etc… i’d advice you to look for a group in the LFG system.
If instead you are totally able to flex and to take roles as solo tank or solo healer, SoloQ is fine for you because you can’t produce any harm to other people.
The wrong way to consider toxicity:
People who complain about toxicity in the game aren’t really complaining about that. Sometime they use toxicity as another way to say: They troll.
Trolling could be: picking a “bad picks” in their view or a general “Gameplay sabotage” kind of behaviour.
This behaviour makes those people angry and toxic as the way it is defined. Insulting and harrassing the people he consider toxic in his opinion because they picked a dps over a main healer for example.
Sometimes toxicity to those who actually are toxic is portraying a team mate as the scapegoat to the problem in a team and the reason for a loss.
This strategy is used to avoid responsability for a loss, augmenting the ego, and a method for not going full committement on a game that as some difficulty. A perfectionist thinking that deny responsability for average game.
About statistics:
In theory to have 100% chance to find in every match a toxic guy is having 1/6 of the playerbas labelled as toxic people.
The problem is that essentialising a guy for being toxic a game isn’t helpful. People become toxic for frustration even if they seemed not to be labelled as a toxic one in general.
So yes we find a lot of toxic behaviour because of people expectations. Generally i think analysing the career could be a good way to say if a person is throwing but it could happen to be his first game as a redeemed new torbjorn only main.
Conclusion:
Generally if you act aggressive, intrusive/invasive and offends in chat or voice, you are toxic even if it’s (in your opinion) best for the match. Toxicity isn’t an huge deal, but when it happens it’s really oppressive.
