....It ALREADY does that.
In all my time of playing TTT and killing idiots who killed other innocents, you STILL lose a minimum of 100-200 karma if you kill somebody even if they already rdm'd a few innocents/detectives.
The karma IS calculated behind the scenes and the effects of it are not applied until the new round starts.
Example:
Player 2 kills player 3 and 4, they are both innocent. Player 2's karma has gone down considerably already BEHIND THE SCENES.
Player 1 kills player 2, player 1 loses 100-200 MINIMUM behind the scenes.
This is done to prevent people from RDM'ing people who killed those innocents based on an INTELLECTUAL ASSUMPTION or WITNESS. Thus preventing them from killing the traitors and giving them a portion of their karma back.
The innocent who killed the one who rdm'd the others will still lose karma, but NOT AS MUCH unless you damaged others. This is what I have read about the TTT info AND experienced in my gameplay.