Why did Hashem refer to the blood of Hevel in the plural ("D'mei")?
Rashi #1 and Targum Onkelus: Because He was referring also to the blood of all Hevel's potential offspring whom Kayin prevented from coming into the world 1 with his murder.
Rashi #2: Because of the many wounds that he inflicted on his body, not knowing on which limb to strike him and kill him. 2
Why does it say that the blood is crying "from the ground"?
Hadar Zekenim : Hashem told him, you cannot say that an animal killed him, for it would not know to cover the blood.