‘Abdullāh ibn Mas‘ūd (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "No soul gets killed wrongfully without the first son of Adam taking guilt for its blood because he was the first to institute the practice of murder."
This Hadīth reports the reason why the first son of Adam will share a part of the guilt or responsibility of all the blood that is spilled subsequent to him. The name of the killer is said to be Qābīl (Cain) and the victim Hābīl (Abel), but this was not reported with an authentic chain of transmission. Qābīl killed his brother Hābīl out of envy towards him. They are the first killer and victim among the children of Adam, so Qābīl is responsible for part of the sin of all the blood that is spilled after him because he was the first to commit murder, and everyone who murders after him has followed his example regardless of the means that they use.