The exile of a manslayer (someone who accidentally kills another) to one of the cities of refuge is for the protection of the manslayer *and* of anyone who might be tempted to bring guilt on himself by taking revenge. See Exodus 21:13.

The wording of this verse ("God let him fall into his hand") implies God caused the death in question, so it wouldn't be appropriate to punish anyone, although some restitution is certainly in order ("life for life"). God gives life and He takes it away. He requires no justification and He answers to no one.