It is plain to see. It is a serial killer, targeting transsexual prostitutes.
The motivations are unclear, it could be that he hates prostitutes, and figures law-enforcement will not take the case as seriously or hunt him down as vigorously, if he picks trans targets.
He could also just be, not hateful of prostitutes or transwomen, and just targeting prostitutes for this reason, for they are easier prey.
However, why he uses a gun is unclear, most serial killers prefer silent weapons, such as knives. Maybe he is using a silencer? The news article does not give enough info to solve the case. It does not explain how they came to the conclusion that he is using a beige car.
Also of note is the racial element, all of his victims seem to be black, when it would have been just as easy to find white prostitutes.
Case 4# is likely a homicide committed by a different person, as the article says.
