Although the killer didn't take the computer, watch, phone, or any credit cards, I'm guessing that no cash was found. I am completely ignorant of how much money would change hands for a sexual encounter, but at a $300 per night hotel, it was probably not a small sum. But still, not likely large enough for somebody's life, you'd hope. What else could Messerschmitt have had in his wallet or on his person that might have been of value to the killer?
The POI in the camera footage looks nervous (pacing), but not enough to abandon the plan when he/she registers the cameras in the stairwell. Also, if any of the fingerprints LE took are from the killer, that's another brazenly confident move, as if there's no fear of any matches turning up.
Either the killer is confident he/she won't be identified locally, or he/she could have been on some serious drugs or perhaps even carrying out some personal mission that is a result of some mental illness or trauma, and DM had some incredibly bad luck in choosing the ad or number that evening.
If only we knew what the killer did take, if anything. A large sum of cash? Killer's contact information in DM's wallet? A second phone used for arranging encounters? It appears the killer was smart enough not to take anything that could show up later, like credit cards and computer, so that makes the theory of drugs or mental illness/trauma not likely, then.
The robbery theory just doesn't add up for me, unless somehow the killer knew DM would have a very large sum of cash on him. No, as I've just rambled through writing this, I've nearly convinced myself that this murder had to be personal -- either for the killer, or somebody who hired the killer.