If someone in the Land kills another accidentally or through negligence, he can flee to one of the cities of refuge until the matter is sorted out or the High Priest dies. If he doesn't run to a city of refuge, then the victim's avenger can assume it wasn't an accident. (Even then, the avenger can't just kill him. There has to be a trial.)
Deuteronomy 19:1-6

In the accidental death in Deut 16:5, there are no witnesses and no evidence to counter the killer's story. Even if it was a deliberate homicide, he couldn't be convicted or punished. Consider how this effects the story of Cain and Abel.