Overwatch when it first came out probably had the best honeymoon period of any shooter that I played. Everyone was super friendly. I remember the moment where I got bit by the Hanzo bug. Enemy team was defending hanamura with a full gun line of bastions. I sneak off to the side and unleash a dragon that kills all of them and we cap. I get the full 11 votes from teammates and enemy for swapping TO Hanzo and countering the enemy. It's surreal looking back at that now and remembering that it was only a year ago. It feels like 10 years ago.
IMO Overwatch's toxicity problem is rooted several major issues.
1. Game lacks proper information conveyance. If you let your users "gap-fill" their own conclusions you create a system where toxicity spirals out of control, because nine times out of ten they gap-fill by blaming teammates.
2. Casual play and competitive play isn't differentiated enough. 6 v 6 makes sense in a comp environment because it's hard to wrangle more players to makes teams with more members. Part of the reason why highlander teams were so janky in TF2 was how difficult it was to get nine people together to practice. But 6 v 6 in quickplay creates a shit environment for fun and experimentation because it has all the individual pressure placed on people from the small team size and none of the competitve incentives to play well because it's "just quickplay." These problems are often exacerbated in arcade mode with even smaller teamsizes. This pressures people to play Comp to get good games. Initially this was the only reason why I even bothered to try comp in first place because quickplay became so shit. Pressuring people to play a mode to play the "real game" is a recipe for toxicity. It would have made far more sense for the regular game to have been 12 v 12 cluster fucks and have tournament mode be the offshoot for enthusiasts. This eases pressure on the meta as well since some characters can thrive in one mode even if they aren't featured in the other. For example, having a symmetra only player in a 12 v 12 pub game would be far more accepted and appreciated than in a 6 v 6 game.
3. Not enough solo play carry potential. It might seem like a weird notion but it is entirely possible for a team based game to be TOO team based. And that is IMO certainly the case with Overwatch. The carrying capacity of individual characters is too low and the need to overly cooperate creates a recipe for blame games. A recipe for toxic behavior. Early on when TF2 was released and you had shit maps like dustbowl and 2fort, and the completely insane launch Demoman; the game basically required multiple medics syncing their ubers together to breach points otherwise you'd never get through the spam. This caused a spike in "team player" servers like the infamous Texas Team Players server which was basically a toxic soup of butthurt where people screamed at you if you put a sentry in the "wrong" place. Over time changes made to the game and unlocks made it so you could be extremely independent. No medic? No problem. Just use the Black box, mad milk, etc. A good player in TF2 could carry a lot of trashcans. In fact most TF2 pubs were basically the top three players versus each other while the surrounding goombas and koopa troopas did their own thing. People deliberately afking as sandvich heavies or spy crabs were just part of the scenery. An afk torb in Overwatch however is a great cause for alarm.
4. The tank-dps-healer paradigm or the holy trinity as it is sometimes called is a shit framework for game design, ESPECIALLY for an action fps game. A lot of people might balk at this, but this system is just awful. People play first person shooters by and large to shooter persons first. Two out of the three core roles are designed around wiping the bottoms of the guys doing the shooting. Not exactly a glamorous job. And yes, there are people who like playing these roles but that motivation springs out of them solely. It's not fostered by the game by any incentive other than "I'd rather not lose." Compare that to other games with supports, like DOTA, where they have their own unique responsibilities and gameplay loops: rotations, stacking, warding, ganking along with their own spells. A Lion having a very good game with blink, forcestaff, euls has a ton of outplay potential compared to a carry like wraithking and PA who are designed around smacking something until it dies, for example. I remember an overwatch game where this mccree screamed at me for not killing a lucio when I was a mercy. The lucio was afk on the cart in Route 66. I was blue beaming the mccree and he missed one full clip of left clicks, a stun grenade (yes he tried to stun a guy who wasn't moving), and a fan the hammer. He did zero damage. But it was my fault even though he was the one shooting. There is NO AMOUNT of health you can stuff in these potatoes to help them. Conversely, in a true moba, even the shittiest carry will get items EVENTUALLY and accidentally attack move into the enemy base EVENTUALLY if you make enough space for them.
5. Bad balance. Self-explanatory and a common problem in games sinces good balance is pretty hard to achieve but like every other point in this list all the other Overwatch problems feed into each other making what individually would be smaller problems much worse. Not enough characters are viable at any one time. The game utterly failed in it's concept of switching to hero counters because the game revolves around mirror match ups. If not exact mirrors, than mirrors of the same strategy. You didn't have reaper comps to counter the tank meta for example, just another tank blob. People get bored of doing the same stuff and want to try new things. Other people don't want other people to try new things. Shitstorm ensues etc. Overwatch is basically the videogame equivalent to those dance movies like Stomp the Yard, where everyone has to dance the same little dance to win the big competition and someone has a breakdown and hides in their treehouse crying into their anime or whatever.
Basically, Overwatch needs. A true fun havey mode that is chaotic enough to mask personality responsibility but still has a general game flow, a greater sense of personal impact and accomplishment, and a more flexible meta that allows people to express themselves while still being competitive. You have all these wonderful characters with their own mechanics but you aren't "allowed" to touch over half of them without getting smacked on the nose with a newspaper because of "reasons" both in game and in the 'metagame' and people wonder why the game is toxic?