99% of toxicity in this game boils down to one thing - Expectations.
-You go in for a gank, and your laner doesn't behave the way you expected.
-You're playing well, but one of your laners is relentlessly feeding.
-You're NOT playing well, and you expect your team to be understanding, and they're not.
-You perform an action, and expect a result, and don't get that result.
The best advice I can give, and what helped me, is to never expect anything from your team mates. You can't control them, you can't advise them, you can't direct them, so why interact with them at all? (in chat, I mean.)
The only thing you should be doing in chat is a "gj" here and there if someone does something cool. That's it. Two letters. All you ever have to type. Anything else serves no purpose, and honestly "GJ" doesn't even serve a purpose - its just a nice thing to do.
But telling people where to go, how to play, or criticizing them, or any other chat behavior is absolutely pointless. I want you to try to think of a time where you said "Hey Jinx, stop feeding" and the Jinx goes "Oh ok, mb" and stops feeding.
Never. If anything, she probably tilted and started feeding even worse. All because you decided it was appropriate to say something about it.
You know what you do instead? Figure out what YOU can do to make the situation better. YOU are the only person you can control. YOU are the only person you should be criticizing or worrying about.
Treat it as if you're playing a coop-vs AI game where its you + 4 bots vs 5 bots. Its entirely up to you to figure out the best course of action to win the game. Everything else is beyond your control. But its better than a coop vs AI game, because if you use smart pings there's a CHANCE the person MIGHT actually listen to you. Bots don't listen to pings, sadly.
But you shouldn't EXPECT the person to listen to you. You're not their boss. You're not their master. They can do whatever they want. They're also in their own little coop VS AI game, and to them, you're one of the bots. They can weigh their options and choose not to assist you - AND THATS OK.
So leave yourself an exit strategy if your team mate doesn't perform the way you expected. "I'm gonna dive this turret and Olaf is going to get the kill."
Ok but what if he doesn't? What if he doesn't even make the attempt? What if he just keeps CSing while you go balls-out under the enemy tower? As of right now, if you're reading this, your response is likely "Flame him for not cooperating and not helping me get the kill, he's bad, he's trash, he should uninstall." But actually the mistake is your own. The play failing, in its entirety, is your fault.
You should have thought to yourself "I'm gonna go under this tower and take 1 tower shot, and then check to see if Olaf is responding. If not, my flash is up, and I'm getting the fuck out."
The first situation has you assuming another player's behavior, and creating an expectation outside of your own capabilities.
The second situation has you only assuming your own actions, and only creating expectations for yourself.
**TL;DR - don't expect anything from other players, and you'll never have anything to flame them for. You are the only person in the game you have any control over.**