@PaulLev @startrek
Don't you think that the reason that the doctor kill the Klingon was because he knew that the Klingon was secretly investing who the federation assassin was to get revenge...?
I mean the Klingon was going around with excuses to meet federation veterans to test their reactions and in the doctor case even test his martial arts skills...
Plus he knew that only the secret assassin and him knew who killed all his men...if he really wanted to defect and promote peace the Klingon could have told the federation about the secret assassin so maybe the federation could keep the assassin away from him...but instead the Klingon kept it all to himself and seems to be doing his own investigation and the doctor felt that if he didn't kill the Klingon it would be too late and in the future the Klingon would assassinate him one day...especially after the doctor begs him to leave him alone...it's was the the Klingon asking the highest in the federation to send all war veterans to meet him too must be...
The scene where the doctor asks the Klingon about Sun Tzu and The Art of War I think shows that the doctor suspects the Klingon of trying to keep his enemies closest...the future knew it was the Klingon that gave the order to torture and kill children when the Klingon lied and put on official record he killed his men because they ordered the tortures etc...
So the doctor knows the Klingon is still capable of anything including these lies in order to find out who tried to assassinate him...so the doctor killing him was just fate since the doctor should have killed him back then...and the broken Medi-Bed at the end was probably broken on purpose so it wouldn't use used to revive the Klingon some how...