I play league and hots and there's toxicity in every gaming community but Riot has started pruning the leaves earlier than Blizzard so I feel like the League community is slightly better in terms of frequency at which I encounter toxicity, but when I meet toxic people in LOL, they're just as toxic as in HOTS.
For example, tonight, I played a game where this Medivh was complaining we were all noobs and that I wasn't healing him. I was playing Lucio, so if people don't know, Lucio heals in an AOE centered around him. Medivh was constantly opening portals and going far away to fight and coming back nearly dead while I was hovering around the other 3 players on our team.
End game, he had the most deaths on our team, second least siege (I was the least as a healer) and second least damage (I was the least as a healer) and the least EXP contributed. He had 0 kills and 13 assists and 6 deaths. I had 3 kills, 26 assists, and 1 death. He also didn't get any medals. I got 41% of team damage healed and got 3 honors from my teammates.
He had literally nothing to complain about. We won and we carried him but he was still calling every single person on our team a noob and complained I didn't heal him yet I had healed 41% of the team damage AND I healed more than the enemy healer.
A bad player on the team screaming and calling everyone else noobs and blaming everyone for their failures.... sound familiar to toxic players you run into on LOL? DOTA2? and every other online game ever?
Personally, I run into them less frequently on LOL even though I play more games of LOL and although that's just anecdotal evidence, it's proof enough for my own edification that Riot's behavior system is going in the direction they intended. It may not be going there as fast as they want or as fast as the community wants, but it's headed in the right direction.