Genesis 4

 It is important to understand that Cain was "cursed" by the Lord saying - 

"...now art thou cursed from the earth, which hath opened her mouth to receive thy brother’s blood from thy hand;

12 When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth."  (Genesis 4:11–12)

Then Cain pleaded - 

"My punishment is greater than I can bear.

14 Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me."  (Genesis 4:13–14)

Then for his protection - 

"...the Lord set a mark upon Cain, lest any finding him should kill him."  (Genesis 4:15)