This would solve ELO hell because your playing with people more compatible to your skill lvl and not people who you are playing against because you got unskilled people on your team in placement matches. I get tired of being 7/5/10 kda but playing with people that are 2/5/10. Assists would also have to play role in this for supports.

Too broken, i steal all the kills and get a zillion elo while my team gets none

Horrible idea. I hate it when people say "OMG MY K/D HIGHER SO I PRO"
You haven't played enough if you believe that K/D matters....

Here's an example. I went 2-11-5 with jax, and maybe around 110 farm in a 30 minute game. Not that great, I know. But my team won by quite a lot (they took zero of our turrets and never got dragon or baron)
The reason for my bad score was because their jungle camped top, so mid and bot lane got ahead. And then in teamfights, I directly jumped on their ezreal, and their team would focus me, and as they were attacking me, my team would be killing them.

I had a bad score, but that is because the other team would rather focus me than our carries. Does that mean I should get less elo?

^ This. I also think this a poor idea. There is no accurate way to apply metrics on situational statistics.