‘And Judah said to his brothers, “What profit do we gain if we kill our brother and conceal his blood? Come and let us sell him to the Ishmaelites, and let not our hand be upon him for he is our brother, our flesh.” And his brothers listened to him.'
There is still disagreement about what is to be done to Joseph. The more belligerent (Simeon and Leah especially? - compare on Genesis 34. They were bloodthirsty enough) still want to kill him while others, like Judah, are probably siding with Reuben's idea. But the sight of the caravan combines the two parties. They can not only get rid of Joseph for good without killing him, but also make a profit out of it as well by selling him to the traders.
“Conceal his blood.” That is, conceal his violent death. But they are all aware that spilled blood, especially of a brother, ‘cries from the ground' (Genesis 4:10).
“Let not our hand be upon him for he is our brother, our flesh.” Judah does not like the idea of killing a person of his own flesh and blood. It would be looked on by all decent people as a heinous crime. Instead they will sell him.