Well, everyone's saying to divorce William, so I'll propose a different viewpoint.

First of all, we don't even know if this really is David. For all we know, it's a prank, or some other evil spirit who is lying to you. On the topic of lying, if it is David, we don't know if what he's saying is true.

He may be lying to you so you will divorce William, since he could be jealous and upset. He also could be mistaken about who killed him, or William could've been framed.

And, even if we assume you spoke to David, and everything he said was true, we don't know why William killed him. Maybe David was going to do something to you, or something to William. Maybe William didn't intentionally kill David.

These are all thing's we can't be sure about. OP, don't dismiss what "David" said, but don't take it as truth either. Do some investigation and report back if you find anything important.