Midrasch zu Bereschit 4:15

And the Lord said unto him: “Therefore, whosoever slayeth Cain, vengeance shall be taken on him sevenfold.” and the Lord set a sign for Cain (Gen. 4:15). The Holy One, blessed be He, said to him: Four families were destined to descend from Abel, but since you have eliminated them from the face of the earth, the earth will open up and devour four families that will descend from you: these are Enoch, Irad, Mehujael, and Methushael.

Cain spake before the Holy One, blessed be He: Sovereign of all the worlds ! "My sin is too great to be borne" (Gen. 4:13), for it has no atonement. This utterance was reckoned to him as repentance, as it is said, "And Cain said unto the Lord, My sin is too great to be borne" (ibid.); further, Cain said before the Holy One, blessed be He: Now will a certain righteous one arise on the earth and mention Thy great Name against me and slay me. What did the Holy One, blessed be He, do? He took one letter from the twenty-two letters, and put (it) upon Cain's arm that he should not be killed, as it is said, "And the Lord appointed a sign for Cain" (Gen. 4:15). The dog which was guarding Abel's flock also guarded his corpse from all the beasts of the field and all the fowl of the heavens. Adam and his helpmate were sitting and weeping and mourning for him, and they did not know what to do (with Abel), for they were unaccustomed to burial. A raven (came), one of its fellow birds was dead (at its side). (The raven) said: I will teach this man what to do. It took its fellow and dug in the earth, hid it and buried it before them. Adam said: Like this raven will I act. He took || the corpse of Abel and dug in the earth and buried it. The Holy One, blessed be He, gave a good reward to the ravens in this world. What reward did He give them? When they bear their young and see that they are white they fly from them, thinking that they are the offspring of a serpent, and the Holy One, blessed be He, gives them their sustenance without lack, as it is said, "Who provideth for the raven his food, when his young ones cry unto God, and wander for lack of meat" (Job 38:41). Moreover, that rain should be given upon the earth (for their sakes), and the Holy One, blessed be He, answers them, as it is said, "He giveth to the beast his food, and to the young ravens which cry" (Ps. 147:9).

"And He said to him: 'therefore, whoever kills Kayin" (Gen. 4:15) - Rabbi Yehudah and Rabbi Nechemiah. Rabbi Yehudah says: the domestic animals, the wild ones and the fowl entered to demand justice for the blood of Hevel. He said to them: therefore I say 'whoever kills Kayin will be killed'. Said Rabbi Levi: the Primordial Snake entered to demand justice for the blood of Hevel.The Holy One of Blessing said to him: therefore I say 'whoever kills Kayin will be killed'. Rabbi Nechemiah says: Kayin's judgment is not like the judgment of the rest of the murderers, Kayin killed and had no one to learn from, from here onwards 'whoever kills Kayin will be killed'. "And Ad-nai put on Kayin a sign" (Gen. 4:15) - Rabbi Yehudah and Rabbi Nechemiah. Rabbi Yehudah said: He made the Sun shine for him. Said Rabbi Nechemiah: to that evil one the Holy One of Blessing would make the sun shine?! Rather, it teaches that He made tzara'at [a scaly skin condition] shine on him! As you say: 'and if they do not believe you or pay heed to the first sign, [they will believe the second.'] (Exodus 4:8) Rav said: He gave him a dog. Aba Yosi ben Keisari said: He made a horn sprout on him. Rav said: He made him an example for murderers. Rabbi Chanin said: He made him an example for ba'alei teshuvah [penitents, returners, Heb. also shavim]. Rabbi Levi in the name of Rabbi Shime'on ben Lakish said: He suspended his judgment until the Flood came and drowned him, as it is written 'and He blotted out all those standing on the face of the land' (Gen. 7:23), just as you say 'And Kayin stood up/rose' (Gen. 4:8)