We are not allowed to take revenge, as the Rambam codifies
This would be, for example if one person did something [against another]; and the other person doesn't stop pursuing him until he pays him back in the same way, or causes him the same pain which he caused. G‑d has prohibited this, in His statement,1 "Do not take revenge."
Yet with goel hadam, the relatives are allowed to seek vengeance and kill the negligent killer (outside the ir miklat). That seems to be incompatible with the commandment of not taking revenge. How can that be?
(I'm asking about the spirit of the commandment, and why there is seemingly that particular exception.)