This will sort out the people who are really feeding and the ones that just do horribly because they're new to the game.

I'm very likely to punish a lvl 30 that goes 7/18/2, but not as much for a lvl 2 summoner.

Even that can be difficult - it would probably be helpful to have timelines included in the chat as to who died and when etc. One team will always lose - towards the end deaths will often snowball once one team gets the initiative or simply starts working better as a team so often the final death count really doesn't help.

Unfortunately as well there's the occasions when a team, or person, is completely outmatched - how many times have their been a couple of level 23-25's matched together against 30's (with 500-1500 wins a piece).

Similarly item buying timelines would be very helpful too.

Unless there is blatant evidence from a player that they are feeding then, on the evidence that is currently presented, then there should be pardons most of the time.