yeah, unfortunately the experience for new players can be very, very bad, to the point that many just quit after before reaching lvl 20. I know, because a friend of mine did just that.
I remember that i received a looooot of flame too when i was low level... i mean i was bad at the game (not that i'm great now, btw), but i was just a newbie with 0 experience with lol or similar games, so i don't think i ever deserved any of that. Heck, i don't see what could i have done to deserving having people tell me to go die or get cancer and similar things.
The reason why i didn't quit is that i don't really care what they tell to me. That's why i never even bothered to mute flamers: every time they offend me it means nothing to me. Maybe it also helps that i'm what you may call a casual player of lol: i play just for fun, i never play ranked nor do i care about ranks in any way. Myabe it's just that i'm too old for it, but i don't give a damn about pro players, nor would i want to be one.
When i play i do my best to win, of course. I also like to cooperate with my teammates and communicate with them. If they are assholes, who cares? i can just ignore what they say. When they go afk is annoying, but well, i'll just try to have fun and hope to have more luck next time.
But even so, seeing people being so toxic is not something i enjoy. I also feel sorry when that happens to new players who are just trying to learn and have to deal with all this shit.
Personally, whenever i happen to play with someone who is still learning and doing bad, i try to put them at ease. If someone is flaming them, i try to get them to stop. However, most of the time when someone is doing bad there will be someone to flame (sometimes the person itself), that's just how lol's community is, i guess...
However, in my experience things get way better once you reach a decent level (at least in normals... i wouldn't know about ranked). When i was low level, the amount of flame was too damn high, but after i reached lvl 30 things changed. It's not like i don't meet flamers anymore, but waaaaaay less than before, and usually they are not THAT bad (as in, they may complain a lot and being annoying, but usually don't tell me to get cancer or wish for me to have a painful death).
So, if you are a new player and you like the game, keep it up: things will probably get better (seriously, i think at levels 10-29 is when you get the maximum amount of flame).