The cops' mission here is NOT to find the homeowner guilty. Unfortunately in some states the law presumes evil intent on the part of the homeowner who acts in self-defense. That's being slowly changed.
No, it's to establish the facts. If the homeowner sterilized the scene, that prevents the police/sheriff from establishing the facts. "Tidying up" or "enhancing" the scene (the "let's drag the deceased back into the house" stuff I've seen a couple of times) is looked at as showing consciousness of guilt.