Is it inevitable that PvP/co-op/multiplayer game communities turn toxic?

Hm, I used to play Puzzle Pirates for 10 year (haven't played it in almost as long now), and in general the community there was very nice.
Of course you had feuds between individuals who didn't get along, but in general a lot of nice cooperative people, and even when you were at war with a different flag and the ships could be sunk, there were rarely hard feelings.

Then again, in the mini-games you had a different set of people depending on which game you played. In the card games, drinking and boxing games people were usually civil and if someone else got lucky they still behaved sportsmanlike. In swordfighting however, some people thought they were the best, and as soon as somebody else won they called him a lucker... odd how the different minigames in the same game attracted a different kind of people.

Without moderation, the worst elements will run rampant (or worse : thrive through means of social memes).
The nature of any individual game can serve as multiplier to this, depending on its design. Small innocent things can have severe negative impact on peoples mood, and if a mood is low enough for long enough, the chances of them indulging their id goes up. Moreso when you consider how multiplayer games tend towards various progression rewards that come a lot faster with wins, and suddenly even your own teammates are ripe targets for self-justified childish sneering. It's fertile ground for trolling.

An example of design choices that panned out badly was All Points Bulletin, which was great for allowing people to make custom paintjobs for their vehicles and clothing, but their idea to create short musical jingles (death themes) that are played to a killed opponent or if you are the MVP at the end of a match was really ill advised. Even well-made innocent death themes become earworms when played as short stingers after every death. Sharp negative gameplay feedback combined with a musical cue, it's kinda pavlovian on its own even before you get into the idea of having a bad match or a losing streak, and then you get people who simply made obnoxiously bad themes, or deliberately made spam themes to be as annoying as possible (someone even found a glitch to make them last for 5 minutes instead of a few seconds). Given it was a "feature", you couldn't turn them off, and it would interrupt any music you were playing even using the in-game custom music player. On top of this, the game used punkbuster, so people became incredibly paranoid about hackers and frequently saw them where there were none (and subsequently a fair slice of the populace carried some form of toggle hack as 'revenge' against anyone they thought were hacking, oblivious to the hypocrisy. :P)

An example of a good choice, that while limiting, was in Heroes of the Storms (MOBA) decision to have in-game chat be team-only, with no global chat. You would still get the usual tryhard 'lifestyle gamer' going into a total meltdown over the smallest of percieved shortcomings, but typically the worst you would get is some passive aggressive jab that goes nowhere.

A properly adapted and designed game will generate far less salt, leaving it more to the bad eggs to stink the place up. A game that hasn't invested time in proper social considerations and lacks high visibility moderation? It may not start too bad, but eventually the unironic douchenozzle mentality will start to seep into it and become more of a norm.

I've been on the Internet for awhile, but it still shocks me that people can type or say certain things to absolute strangers and feel justified in doing that

Hm that is something to think about. I can't give a definite answer. Sometimes when I read something horrible on the internet I either ignore it or just think "well this is an idiot" and go my own way. And sometimes I'm genuinely puzzled and try to imagine what they're going through, are they sad or why are they acting like that. It depends on how I feel at that moment.

I think most of that is subjective. I played a lot of online games and I can count on one hand how many times I've come across on some idiots. I suppose I always had the luck to be grouped with decent people online. Some people are like garbage trucks. They run around full of garbage, full of frustration, full of anger, and full of disappointment. As their garbage piles up, they need a place to dump it and sometimes they'll dump it on you. Don't take it personally. Just wave, wish them well, and move on

My opinion on the matter is a very simple one. I think people fail to realize that toxic players are almost always a minority, just an extremely VOCAL minority. Riot released some numbers a few years back showing that only about 5% of players have exhibited toxic behavior and yet people will swear that it FEELS much worse than that. Well 5% means 1 in 20 players. With 10 people per game, that means you're likely to run into a toxic player in every other game. So it's not the community itself that became toxic, it's that the bad experiences get blown WAY out of proportion.

There will be toxic players in certain kind of games when playing games like mmo´s or moba´s in random groups but to say that its inevitable that co-op multiplayer gaming communities always will turn toxic i disagree.

I like playing co-op myself and i enjoy my time playing co-op games with the After Party Group for example

we pick a player of the week every week and he can suggest games to play for that week and its up to the people if they want to join or not
we tend to play co-op games on wednesday's friday's and Saturday's

Not always. It depends on the age of the average gamer playing the game.
If its something younger people play, then yes, it tends to get toxic quick. If its something geared at older people like Age of Empires online, then it was very pleasant, and I never came across even a single toxic person. I think the average gamer for that game was like 30-40 based on the people I talked to. Contrast this to something like rust, and you will see a stark difference.

For me playing games co op is important even if it can get toxic is because I have made some good friends from those games. Now I just play co op games with them in my team, so less chances of running into toxic players. As time goes on, you collect more and more non toxic players to play with, and its a lot more fun playing with team members than alone or against them. Yes, it means there are more points of failure, and you can lose due to someone else's inability to play well, but that is the beauty of co op gameplay. Instead of just focusing on your own game, you get to play well in a team. If you reach adulthood and struggle to play well in a team, you are probably gonna struggle. This is also why adults are less likely to be toxic in games as they have likely experienced that in real life.