I could pull up numerous crimes right now which begun as "armed" robberies and ended up as murders, that's why they're called "botched" robberies. David was a lawyer; if he threatened the person who was attempting to rob him, maybe that person decided they couldn't risk it--probably a seasoned criminal, maybe on parole--and just quickly reacted by stabbing him. By then, David had already seen his or her face--they couldn't just "leave him there", so to speak.
Another motive could be sexual sadism. The person on tape could be lesbian or transexual. It's not necessarily just a woman "pretending" to be a man. Crime, in general, doesn't make sense. People always want to find a deep motive or some sort of big story; often, it's just a case of a person being in the wrong place at the wrong time. Also, there's a reason why drugs and crime go together like bread and butter.
I read that the victim was found partially clothed. If this begun as a sexual encounter, it's entirely possible the victim was defenseless, laying on his stomach. He could've also been stabbed walking towards the door or turning repeatedly and quickly, fallen on his stomach or kneeling down and stabbed again for "good measure".