Ramban

AND WE SHALL CONCEAL HIS BLOOD. “We shall hide the fact of his death.” This is Rashi’s language. And Onkelos similarly says, “and we shall cover up his blood.” The correct interpretation is as its literal sense indicates. It is the custom of those who kill in secret to slay the victim, bury him, and conceal his blood in the earth, even as it says, And he hid him in the sand. This was why Judah said to them, “By casting him into the pit we shall kill our brother and cover his blood with dust, for it will so be accounted to us.” Now Reuben had instructed them not to spill blood with their hands. Rather, they should throw him into the pit and let him perish there, since the punishment of he who causes bloodshed is not the same as the punishment of one who actually commits the murder. Judah now came and said, “This too will be accounted to us as murder, as if we had killed him.” Such indeed is the truth, as the verse says, And him [Uriah] thou hast slain with the sword of the children of Ammon. The difference between actual murder and causing death is that there is a greater punishment for a murderer and a lesser punishment for the one who indirectly causes death. Thus, the two of them [Reuben and Judah] spoke the truth.