There's a few things going on.
Generally speaking, most people who play league of legends are decent, not flaming people. Sometimes people get a little frustrated, and sometimes people have a bad game. But only a small fraction will rage and flame.
There are several factors that contribute to exacerbating the problems.
- League is team reliant and competitive. Your success is only 20% of the picture (10% if you consider the other team too). Most of the game is out of any players' hands. This is frustrating because there's a bad correlation between skill and winning. In any solo game, skill has a strong correlation to winning.
- Negativity bias: It only takes one negative player to make the game experience negative. Even if the other nine are decent, you remember the one who wasn't.
- League's leveling system is clumsy and archaic (hopefully this is no longer the case, with the preseason changes). In the early days of league, the leveling system matched players of similar level, which due to runes and masteries, mattered. But since then, the vast majority of players have reached level 30.
This makes low levels (1-29) a selection of new players, smurfs, and most significantly to your experience, banned players on a new account. The rate of toxic players is disproportionately high at low levels. People afk far more frequently at low levels. And because some a fair fraction of players are still learning, people can feed hard.
My hope that, with the changed leveling and rune system, much of the new player experience problems go away. Without having to worry about summoner level matching, Riot can put new players with less skilled experienced players (There are plenty of bad players in league. Have you seen any of Kshaway's wood league videos?)
There's very little advice I can offer you, though, besides the classic advice of 'mute, report, and move on'. The new player experience is pretty bad in league (as you have noticed) and before this new season, the only advice was "it gets better at level 30". I really hope Riot takes this opportunity to adjust new player matchmaking, but I'm not certain anything has changed.