Gang initiation might sound a little far-fetched, but that's the scenario I keep going back to.
The only thing is, DM wasn't making any attempt to look or act the part - what kind of gangster wears a satchel and plaid shirt? And MS living at home with his Mom, dutifully walking the dog.
I wonder if it was some sort of private fantasy between them. Folie a deux, as someone said.