I disagree, I think a slap on the hand wil work fine.
A TK penalty might make the archer think twice about shooting at an enemy that is about to get backstabbed.
No, it won't. And no, it won't, unless the punishment is scary. Also, some of the most epic moments in this game occur when people take risks.
EXAMPLE: My teammate was in a fight 1v1, and won. Standing victorious, he was facing in the direction where most of the enemies were located. However, a ninja was creeping up behind him to stab him in the back. He looked at me, and it appeared I had my Xbow aimed straight at him. He began to raise his shield just as I fired a bolt over his shoulder. BOOM, HEADSHOT! The dishonorable ninja died, and I saved my teammates life.
If there were such a steep penalty in place that it was scary to take that shot, such epic moments would cease to exist. And if the penalty is only slight, then it will make no difference, except that devs put time into the code for nothing.
Also, I replied to your other post by editing it into the post above.
EDIT: A little background info on Heroin: When I started playing cRPG, I was a complete noob to warband in general. I had only owned the game for a week. When I was playing in my first couple days, a guy wasn't careful an TKed me. As I had seen most people in the game apologize when they TKed someone, this guy didn't. So I said something to the effect of, "No apology?" The guy proceeded to kill me at spawn every single time I spawned for the next 2 weeks. Once I had learned to play, and semi-competently defend myself, he had his buddy join in with him to TK me. It was so bad that I could not even play the game if these two guys were on. And any time I said anything about it on chat, they would accuse me of attacking them, to make me look like the bad guy to the public/admins. I recall spending HOURS being TKed over and over by those guys, with nothing I could do about it, until I discovered the forums. That entire situation happened under the old-school XP penalty that you're asking for.