Why do diamond players deliberately tank smurfs down to bronze/silver/gold?
Why do so many D1+ players engage in elo boosting?
Why are there so many leveling bots in ARAM/bot games?
Why do high elo players like Pornstar Zilean put so much effort into trolling despite being better than 99.99% of the community?
Riot wants people to live, eat and breath their product. The fact that you have to play hundreds of matches to reach level 30 and accumulate enough IP for a decent suite of rune pages denotes the sort of time that Riot expects its playerbase to dedicate to League of Legends. You also have to play at least once a month or your account decays in plat+. So people are sucked in by the hype, the eSports culture, the reddit drama. They get in the habit of playing LoL whenever they can - they have to if they want end of season rewards, they even have to play a significant number of matches in multiple queues to get all of them. It becomes part of their life and they get emotionally involved in their games. They rage, they flame and they abuse. Yet they get home from school/work/uni the next day and do it all over again.
Then when they finally reach D1/Master/Challenger they've made it. They're at the top of the ranked ladders. They're the big swinging dicks of league of legends. But do you know what? It's actually kind of boring. They're playing against the same people every game and there are very few surprises. There's that guy who only plays Katarina, that guy who mains Vayne and makes dubstep frag movies that all look the same, a fuckload of ADC mains who spam whatever is OP at the time. It's repetitive and, moreover, there's nowhere to go from there. They can look at joining a pro team but they don't have the time to dedicate to professional play and there's fuck all money in it anyway.
So they look for other ways to enjoy themselves within the confines of the game that they've dedicated so many hours of their lives to. They buy a fresh level 30 account off ebay for a minimal amount of money (obviously, who would waste another few hundred games leveling a smurf themselves?), buy some champs and IP boosts with RP (Rito says thanks very much for the $$) and throw their placements so they end up in low elo. They play troll builds and still win then play disco nunu to tank their MMR when they're near promo. The game regains some level of appeal for a while. Eventually they get banned and repeat the whole process. This time they climb up to high elo and, since it's a smurf, they troll high elo players they don't like by throwing when they're on the same team and tryharding when they're up against each other. There are a lot of angry kids in high elo so it's pretty easy picking.
Meanwhile, the company that our archetypal high elo troll buys fresh level 30 accounts from also does elo boosting. They offer him $100-500 per job to do what he does anyway: play league of legends. He accepts, the money is better than "professional" players in Australia earn.
I'm not saying it's Riot's fault that players troll but it's tremendously short-sighted to create a game requiring so much time commitment that it becomes part of the daily routine of its playerbase and not expect some backlash when those players lose interest. The word "toxic" is thrown around so much that it's lost all meaning. The only truly toxic thing in league of legends is the game itself. It worms its way into the life of its players, most of whom are teenagers and young adults who lack the judgment and self control to limit their gaming time, and constantly demands attention. Sure, it's hugely enjoyable but it singularly discourages moderation.