Its a sign of remorse. If you have ever read any of John Douglas' books, or even "whoever fights monsters" by Ressler, they go pretty deep into. When a serial killer or murderer in general covers up a body with an object like that, its typically a Sign of remorse. It would make sense in this case too. They had to kill him, not only to move up in chosen profession, but also to not be killed themselves. But this is the guy who showed them around, they went to his place for barbecues and camping. I imagine theyhad a pretty close relationship with the guy. But you have to do what you have to do.