Parasht Vayeishev 5772: “... and dipped the tunic in the blood”

Then they took Joseph’s tunic, slaughtered a kid, and dipped the tunic in the blood. ...He recognized it, and said, “My son’s tunic! A savage beast devoured him! Joseph was torn by a beast!”

Why did the brothers dip the tunic in goat blood? Does goat blood resemble that of a human being, as Rashi remarks? Who is the savage beast that devoured Joseph?