NOTES for Deu 19:1-21
The question of cities of refuge is raised more than once in the Torah, and evidently it is extremely important. This is no surprise: the matter concerns killing and the punishment for it. The Torah, and Deuteronomy in particular, treats the question of crime and punishment, and of the investigation of crime, with extreme seriousness. Every accusation must be proven and supported by witness testimony, and when the crime is punishable by death, the testimony of at least two witnesses is needed, and preferably, for reliability, three.
The point here is not only that one witness is easier to bribe or intimidate than two or three. It is also that cross-testimony always reconstructs a more objective picture of what happened, if only because it allows the same events to be seen through the eyes of different people, and therefore from different sides and different points of view. The picture of what happened becomes three-dimensional and many-sided. All this was especially important in investigating a crime such as murder. In a traditional clan-based society, the mechanisms of blood revenge usually came into play here, and behind them stood a powerful layer of the most ancient, semi-magical beliefs.
Murder, the shedding of blood, was considered defilement in itself, regardless of whether the killing was a violation of the law or not. Even the killing of an enemy in war defiled the killer; it was not considered a crime, but it still required purification. This was even more the case when the killing of an innocent person was involved: such a killing required cleansing by blood, the blood of the murderer. Not to avenge spilled blood meant becoming an indirect accomplice to murder, if, of course, vengeance was possible.
It is precisely this magical way of thinking that Deuteronomy commands people to leave behind when investigating murder. Traditional ideas had to be set aside, and attention had to be paid only to whether the killing was accidental or intentional, deliberate or spontaneous. This is understandable: a person's actions matter to God, but the intentions behind them matter no less. Everything else is unimportant to Him, and the Torah commands attention only to what is important to God.