"Thou shalt not kill." (Exodus 20:13)
The basis for murder is hatred (Deuteronomy 19:11; Exodus 21:14). The use of a weapon identifies murder. It may be a lethal weapon, like an "instrument of iron" (Numbers 35:16); a weapon of opportunity, like a stone or club (Numbers 35:17-18); or merely the use of hands (Numbers 35:21).
In contrast, accidental killing is distinguished from murder (Exodus 21:13). Sometimes identified as "unaware" killing (Deuteronomy 4:42) and described as "error" killing (Numbers 35:11), it occurs without enmity (Numbers 35:22; Deuteronomy 19:14; Joshua 20:5) and by accident (Numbers 35:23), even though it may result from carelessness (Deuteronomy 19:5).
Execution is demanded for premeditated and presumptuous murders. The original authority was given to corporate man by God after the Flood (Genesis 9:5-6) The process of trial and conviction was established in Numbers 35:30-31. All such laws are designed to suppress evil (1 Timothy 1:8-10).
Imprisonment from normal society is demanded for accidental killings. Cities of refuge were built for such manslayers (Joshua 20:1-9) and were to be easily accessible to the nation (Deuteronomy 19:7-8). They were places of protection (Numbers 35:15) and restriction (Numbers 35:26-28), to be voluntarily entered (Exodus 21:13; Numbers 35:11). Imprisonment was for an indefinite length, and a person remained in the refuge until the "death of the high priest" (Joshua 20:6).
Modern laws dimly reflect these ideals but are made less effective by delay. "Because sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil" (Ecclesiastes 8:11). HMM III