I DO NOT CONDONE NOR ENDORSE TOXIC BEHAVIOR IN ANY WAY SHAPE OR FORM, however, As a resident toxic player, I'm going to ask you a simple question: Why shouldn't people be toxic. League of Legends while being a team game no doubt, it puts an extreme focus on individual performance. Often times at almost every elo except the absolute highest, which is something less than 1% of the player base are in, the game feels as if it is a single player game with 5 players stuck in the same game trying to win it in their own individual way. While this obviously is not how the game is supposed to be played, with a meta in which the game can be snowballed and ended by 20 minutes its difficult not to think this way. "I play for my advantage" is a common sentiments among all players in this environment. When people make the process of doing well and winning more difficult the natural response is anger. Getting angry at someone online is no different than getting angry at someone who missteps in the real world, except here is the crucial part: anonymity. The worst punishment you could possibly receive for flaming someone else is a ban, however, that's extremely unlikely to happen in the majority of situations. Thus without fear of punishment, the stress of attempting to win a competitive game, and a myriad of external stressors that players cannot be handle in the same visceral manner, it only makes sense that people would lash out. While in no way am I condoning this activity, let's not be naive and assume it doesn't make sense. People are angry. People lash out when they are angry. The only thing preventing people from lashing out is through punishment. Don't establish a narrative that being angry or toxic is somehow additional work. In a world surrounded by deadlines and stressors, people are increasingly angry. “Half of all Americans are angrier today than they were a year ago,” while only 8 percent say they are less angry than they were last year." -The American Rage, a survey conducted utilizing 3,000 Americans found in January of last year. https://www.esquire.com/news-politics/a40693/american-rage-nbc-survey/
Thus the real question should be, what makes people not angry. Humans have an automatic predisposition to aggression when adrenaline is pumping, so thus anger is our "natural" state when playing competitive games.
tldr: People are naturally angry and in a meta that prioritizes early individual leads, it's extremely easy to get upset at someone messing up.