let me mid or i go feed (in ranked)

It is your choice. In my experience, 90% of these types of players do not actually end up feeding in game, and I have not experienced different win rates between games like this and your average game. Thus, I tend to just say "no, I'm going to keep my assigned role." and remind them that abandoning their role/refusing to play their assigned role is punishable, as is intentionally feeding. If they still go through with it? A report will virtually certainly result in an instant 14 day ban on their account. If they don't go through with it, no harm no foul.

Flame riot for making a shitty system for ranked

"do it m8 you wont"

^ me every time

[katarina-love]

I would play it out, and report them for pre-game chat, regardless of how they play, adding intentional feeding if they do so.

send a ticket to riot. they'll do what they do. but they will never reveal the outcome of your ticket or what actions they taken.

I would take screenshots when they state their intent & explain what they did.

Do not engage them for their behaviour or threaten them.

You may lose LP for their actions, but they may lose access to the account.

Let him feed and report him after the game.