Ranked too harsh. (Salty af)

This might just be me being the salt king but after my last ranked game I need to complain.

So I get in a game alls good for the first 20ish minutes the game is tied me and 3 of my teammates get into a teamfight I as the assassin head straight for the back lane and start taking out the adc and mid laner I one shot the adc and the mid laner gets away with a little bit of hp left our 4th teammate who was top clearing a wave that had at least 1 and a half minutes till it reach a tower comes mid where the fight was taking place sees the half dead lux and goes in ofc I am already there I kill her then head back to the teamfight. Next thing I hear in chat "Akali ks!" Then in all chat "Please report akali for troll and being toxic" I'm just there like "What??" ofc the game goes on from there however the 4th teammate is just done he doesn't help and every time he dies he blames me for causing him to be "Behind" He was 6/6/12 one of the best in the game... We lose a teamfight they push mid I say in chat "You got this!" Nope he goes in doesn't ult (Tryn) throwing the game by a one second next thing I see in the after game chat "Akali took all the kills and didn't carry please report." And his friend I guess?? "Akali enjoy elo hell!!" All because I didn't give him a kill he in no way deserved...

But the real problem and reason for this thread is I went from 20 LP to 1 LP... Just got promoted and was about to be demoted because I know how to play... I feel riot should not punish people so hard for knowing how to play... But whatever this is just me being salty... Though it does feel good to get it off my chest so to say.

Ofc you know something I could have done besides carry a team who doesn't even try please let me know I am sure this is in no way the last time.

One more thing is it just me or should the amount of LP you lose depend on the game because I feel 19 is way too much for such a close game... i mean we were at their nexus!