And the Lord said to Cain: Where is thy brother Abel? And he answered, I know not: am I my brother's keeper?
And he said to him: What hast thou done? the voice of thy brother's blood crieth to me from the earth.
Now, therefore, cursed shalt thou be upon the earth, which hath opened her mouth and received the blood of thy brother at thy hand,
When thou shalt till it, it shall not yield to thee its fruit: a fugitive and vagabond shalt thou be upon the earth.
And Cain said to the Lord: My iniquity is greater than that I may deserve pardon.
Behold thou dost cast me out this day from the face of the earth, and I shall be hidden from thy face, and I shall be a vagabond and a fugitive on the earth: everyone, therefore, that findeth me, shall kill me.
And the Lord said to him: No, it shall not be so: but whosoever shall kill Cain, shall be punished sevenfold. And the Lord set a mark upon Cain, that whosoever found him should not kill him.
I've come to the conclusion that asking, "Why?" in reference to the Old Testament god's reasoning for doing anything is about as fruitful as trying to rationally inquire as to why a schizophrenic believes there is a gps tracker in his ribcage.