Constable Ken figured it for a suicide after he found a note in the pocket of Benjamin’s bloodstained jacket,
which he’d flung across the room.

However, if Benjamin had cut his own throat had been cut so viciously that his head was almost severed he wouldn’t still have then been able to still take off his jacket and tie off afterwards and fling them across the room.

No, the jacket and tie must have been flung across the room by somebody else. And since Bonnie was the only one in the house, Field-Marshal Mogau’s detective eyes fell on her.