I've been struggling to find answers as well and talk about it; but the fact is that people are anonymous (or so think) and think they can behave however they want to. Its not only in LoL or games, but internet overall where people are horrendous many times. I even got a person to jump into my stream and say "You are a fat red head, go and shoot yourself" and other threats.
And not to mention some people do it because they think its fun to make other suffer and rage, the so called trolls. Had a two games a while ago where I was playing Bard Support, and there was a guy that I got in the team two times in a row, and he complained and turned the rest of the team against me for pretty much doing my job and being Bard (which is a roaming support) etc. It pretty much made me stop playing for a few days.
Also people are quicker to judge and call out a mistake than a good play aka if you make a mistake they yell and curse you but if you do something great then you rarely get praised cause its your 'job' to do that. Its an evil spiral where people make each other angry and give birth to more anger and toxic. I know many blame the game companies for it, that they need to fix it but its fact, its people that need to change not Riot, Blizzard or whatever.
All in all is it a huge thing, and probably impossible thing to change, though as long there is people out there to speak to the younger and coming generation is there some hope. : )
