I'm going to leave poor Bowie out of these thoughts, since as far as I know, no cause of his death has ever been officially released.
I think the point where I veer away from believing that Katherine was murdered by some random, psychotic stranger who lost it one night in a dark park is around the personalization, the act of carving the word "fat" into Katherne's chest, and the time that took.
I can envision a frenzied cyclone of stabs raining down on Katherine's body - much as I could envision Kohberger's surging frenzy as he killed his four victims, only one of whom managed to fight back. But here, IMO, between abled-bodied Katherine, her dog, the possibility of passers-by in a public park, etc., the struggle must have intense, frenetic and noisy, albeit brief, until it wasn't. Once Katherine was down and dying/dead, I feel like most assailants would have fled the scene immediately, especially if the killing were spontaneous, the result of some kind of psychotic break.
The choice to carve the word "fat" into Katherine's body seems to have been important, because, IMO, every second the killer remained at the scene to accomplish that, the risk of discovery was exponentially increasing.
I'm not sure who the word referred to, Katherine or her killer, but the presence of it personalizes this murder, IMO.