Jared Free - Decalogue "Thou Shalt Not Kill"

In discussing this film I found it hard to determine which death was the greater sin. From a cold and literal perspective, the death of the cab driver was the greatest and only sin. From a legal standpoint he was an innocent victim. From a moral perspective, however, he was significantly flawed and was a man without any redeemable qualities; at least none that the audience was made aware of. Does this make his death, from a religious standpoint, justifiable? or sinless?
The murderer himself was killed. Was his death a sin? I felt that the death of the murderer was more tragic than the death of the cab driver. The murderer was a young man, he had lost the one thing in the world he loved most and had sentenced himself to a life of solitude and loneliness by running away from home, there was no warmth in his life save for one occasion when he was able to briefly interact with two young girls.
Both deaths were calculated and deliberate. Even though only one was unjust from a legal perspective, from a religious perspective both were sins. If there is a degree to which something can be a sin (a minor sin or a major one), however, than which one was more severe?