You mean married men couldn't be murdered?
Of course they could be murdered. But when Seth was born, Eve said "For God, said she, hath appointed me another seed instead of Abel, whom Cain slew."
So, if Abel was married, he had no sons. When Cain slew Abel, Satan was attempting to destroy the Seed line from which the Seed would come who would destroy him. (Gen. 3:15) First prophecy in the Bible upon which all other prophecy is built.
Quantrill