Берешит 4:10 Комментарий: Rashi, Kitzur Ba'al HaTurim, Radak & Rabbeinu Bahya

דמי אחיך THY BROTHERS BLOOD — דמי is plural — bloods” — his blood and the blood of his possible descendants (Genesis Rabbah 22:9). Another explanation of why the plural is used: he inflicted upon him many wounds, because he knew not whence his soul would depart (i. e. which blow would prove fatal) (Sanhedrin 37b).

"Crying out" - In one version of the tradition it says "and also they cry out..." Rashi explained there that it teaches that he (Pharaoh) contracted leprosy and was slaughtering Jewish infants and washing himself with their blood, and this (the infants' blood) is the voice of your brother's blood crying out. (Genesis 4:10)

ויאמר...דמי אחיך, He said: “the blood of your brother whom you have slain cries out to Me from the very earth, and the sound of its cry is what I have heard and what caused Me to confront you.” This was, of course, a figure of speech. מן האדמה, upon which you spilled his blood. The fact that the Torah uses the plural mode when referring to blood, writing דמי instead of דם, is not especially significant, as the word דם occurs frequently in the singular mode and also in the plural mode. (Compare Leviticus 20,9 and Leviticus 20,11 to mention just as few examples.) Actually, the justification for treating the word דם as both singular and plural is the fact that on the one hand it is an entity, but at the same time it contains 4 different categories of moisture. Onkelos explains that the reason why here in particular the use of the plural mode is appropriate is the fact that by killing Hevel who had not yet been married and sired children, spilling his blood also meant spilling the blood of the children he would now never have. He may not even have had marital relations with his wife as yet, just as his brother Kayin had also not yet had marital relations with his wife until after he had murdered Hevel (comparer 4,17).