Deuteronomy 19:6

6 Otherwise, the avenger of blood might pursue him in a rage, overtake him if the distance is too great, and kill him even though he is not deserving of death, since he did it to his neighbor without malice aforethought.

Read Deuteronomy 19:6 Using Other Translations

Lest the avenger of the blood pursue the slayer, while his heart is hot, and overtake him, because the way is long, and slay him; whereas he was not worthy of death, inasmuch as he hated him not in time past.
lest the avenger of blood in hot anger pursue the manslayer and overtake him, because the way is long, and strike him fatally, though the man did not deserve to die, since he had not hated his neighbor in the past.
“If the distance to the nearest city of refuge is too far, an enraged avenger might be able to chase down and kill the person who caused the death. Then the slayer would die unfairly, since he had never shown hostility toward the person who died.