I have wondered if it was someone who knew Katherine and Bowie. Someone Katherine would not have been creeped out by, and Bowie was friendly with...
The most horrific attacks are usually very personal, a rage killing. And it sounds like both Katherine and Bowie were "overkilled". With the body mutilation that has been mentioned by LEO. Although, the details have been vague.
Who kills a dog? Especially a pittie? And who was killed first? If the perpetrator had killed Bowie first, Katherine would have screamed? Run away? So, if Katherine was killed first, Bowie may have been trying to protect her...and was subsequently killed. Seems almost premeditated. Not many people would attack a woman with a large dog.