When you checked and entered each guestroom, other than you and the respective cousin, no human existed within the room.
Culprits cannot die, jet you discovered your cousin Bills body (there are red truth for that). I thought that the owner may be a cousin of yours as well, but he is in his own room. That can only mean, that the body of your cousin Bill was stabbed and bloody, but he wasn't actually dead!
-The body which you discovered in Bill's room belongs to Bill. The body must really be your cousin bills, since you didn't enter the mansion or owner bills room at any time, and therefor couldn't identify owner bill in his own room.
At first it sounds like that one:
- Each of the three bodies you discovered was unmistakably dead.
*This is at the time which you discovered each body. makes everything an logical error, since culprits cannot die. but at some time, after the police arrived and carried the dead body of owner bill outside (whom you hopefully called) you had to discover the owners dead body as well. So you discovered 3 bodies, and each was dead. But actually, you discovered 4 bodies, and 3 of them were dead. I thought it was some poison which lets you be dead for about a week or so, but since a culprit cannot die and therefor cannot be "unmistakably dead" at any moment of the story, this option isn't valid.
I was under the impression that you had reasoned that far already.
So I'll play along. I brought him back to life with magic.
Not valid. culprits cannot die at any point of the story. Which means he couldn't die and therefor couldn't have been brought back to life...