For me it was probably the trolls, afkers and feeders who were in my team and trolled the game. Seeing them go unpunished despite reports after ruining the game and waste 2 hours of my time (it takes 2 extra won games to get back to the elo where I should be given nobody trolled the first game) just feels so disappointing and frustrating. I played since S2 and I reached diamond in S3, however I was not good enough player to hard carry every game in diamond, and I couldn't just shrug off those occcasional troll games that made me lose valuable points of elo for no good reason. First I became mildly toxic and eventually got restricted last season.
Verbal abuse, while not nice, was still easy to shrug off since the mute button exists and it doesn't directly cause elo loss. I'm pretty sure giving harsh punishments for verbal abuse while letting the trolls have it easy is the worst way to handle the toxicity in this game. Trolling is a major reason why verbal abuse level in the game is so high. Giving a punishment to somebody who flamed a troll and let the troll walk free will result in a extremely resentful environment.