פּירוש אויף שמות 21:12

מכה איש ומת WHOSOEVER SMITETH A MAN SO THAT HE DIE [SHALL SURELY BE PUT TO DEATH] — Many apparently redundant verses have been written in various sections of the Torah dealing with murderers and I shall explain to the best of my ability why all these statements have been made.

If one strikes a man and he dies. It does not say that he “killed” him, but only that he “struck” him and he “died.” The implication is that a murderer is merely Hashem’s agent to execute judgment upon one who has done something to deserve it. Nevertheless, the murderer also earns the death penalty by his deed. He shall be put to death (literally, “Death, he shall be caused to die”). The murderer has incurred eternal death. Therefore it is an act of kindness to cause him to die in this world so that he can be restored in the next.

From My altar. In other words, one should not think that he should be left alive to make amends and to perform good deeds in the future, because Hashem does not wish to receive anything from him.