Why does the punishment of him who kills Cain seem more severe in Genesis 4:15?
God wanted to nip the fast multiplication of murders in the bud.
Calvin's Commentary on the Bible
But this sentence ought to be read continuously, thus, ‘Whosoever killeth Cain, shall on this account, be punished sevenfold.’ And the causal particle לכן (lekon,) indicates that God would take care to prevent any one from easily breaking in upon him to destroy him; not because God would institute a privilege in favor of the murderer, or would hearken to his prayers but because he would consult for posterity, in order to the preservation of human life. The order of nature had been awfully violated; what might be expected to happen in future, when the wickedness and audacity of man should increase, unless the fury of others had been restrained by a violent hand? For we know what pestilent and deadly poison Satan presents to us in evil examples, if a remedy be not speedily applied. Therefore, the Lord declares, if any will imitate Cain, not only shall they have no excuse in his example, but shall be more grievously tormented; because they ought, in his person, to perceive how detestable is their wickedness in the sight of God. Wherefore, they are greatly deceived who suppose that the anger of God is mitigated when men can plead custom as an excuse for sinning; whereas it is from that cause the more inflamed.
God wanted to be in control instead of men.
It is mine to avenge; I will repay.
Keil & Delitzsch Commentary on the Old Testament
God granted him continuance of life, not because banishment from the place of God's presence was the greatest possible punishment, or because the preservation of the human race required at that time that the lives of individuals should be spared, - for God afterwards destroyed the whole human race, with the exception of one family, - but partly because the tares were to grow with the wheat, and sin develop itself to its utmost extent, partly also because from the very first God determined to take punishment into His own hands, and protect human life from the passion and wilfulness of human vengeance.