The problem I see is that there is no way to differentiate between legitimate accidents, which I don't see as a problem, and plain old carelessness or intentional killing.
Mistakes happen - it really doesn't bother me if someone just accidently kills me. I always say I'm sorry when I do it, and I always accept apologies no matter how dumb the mistake was.
But, what bothers me is for instance, in the middle of a 1 v1 melee fight - you hit the guy twice and right before you finish him off a 'friendly' archer shoots into the fight, hitting you in the back and killing you, just hoping to get a lucky and grab a killshot.
Or, some idiot with a sword jumps in the middle of the fight at the same time and causes you to hit them, and instead of one of you killing the guy when he was vulnerable, both of you end up dead just because they wanted to get the killshot. Then the jerk gets mad at you for his own careless, greedy mistake and revenge tk you, double screwing you over.
Or if I was cavalry, someone hitting my horse for no reason would bother me as much as when 'friendly' cavalry charges over me, knocks me down and causes me to die (these two behaviors might have a positive correlation).
If there were an automated way to distinguish between these team hits and true accidents, then this idea would be feasible, but the only way to do that is case by case, and that is what they do currently through the use of admins.