I've found documented cases where people have inadvertently altered a crime scene, but I can find no case where someone has intentionally removed a weapon. That's just not how people behave, and if you listen to the 911 call, it's clear that the caller was a mess. Such a mess in fact that he was unwilling to get too close to ST's body, and doesn't appear to have seen the body of his wife.
In some alternate universe where that did happen, this person would have almost certainly fessed up to it by now.
Just because a weapon is missing though, police don't (or aren't supposed to), immediately determine a case to be straight up murder.
There's gunshot residue, wound bullet trajectory, blood spatter, autopsy findings, and scene reconstruction.