Jamieson, Fausset and Brown on Genesis 4:15

15. whosoever slayeth Cain--By a special act of divine forbearance, the life of Cain was to be spared in the then small state of the human race.

set a mark--not any visible mark or brand on his forehead, but some sign or token of assurance that his life would be preserved. This sign is thought by the best writers to have been a wild ferocity of aspect that rendered him an object of universal horror and avoidance.