Always have a penalty would force people to play smarter. Example: not swinging wildly side to side in a group of people, or archers not firing into a large group of friendly melee
HOWEVER, I could see this being abused by people getting in the way of others that they do not like
why always ? most of TK happens in moment N people attacks one target and the target dies
the corpse instantly turns into 'thin air ghost' (because of lame ragdoll system applied to corpse)
and the attack some someone from N group usually teamkills someone from own team ...
that's definitely not on purpose or 'not being careful enough'
for this reason the 'forgive' system is best because the victim decides if it was the case
and usually well said apologize proves it ... (people need learn to apologize)
it works in many FPS and RPG / MMO games ... why not in Warband
another problem is 'damage / repair' caused by deaths from TeamKills ... can it be removed?