John Gill Commentary Genesis 37:31

"And they took Joseph`s coat, and killed a he-goat, and dipped the coat in the blood;" — Genesis 37:31 (ASV)

And they took Joseph's coat After they had told Reuben what they had done with him, who being willing to make the best of things as it was, joined with them in the following scheme: by this it appears, that when they took Joseph out of the pit they did not put his coat on him, but sold him naked, or almost so, to the merchants:

and killed a kid of the goats, and dipped the coat in the blood; that being, as the Targum of Jonathan and Jarchi observe, most like to human blood.