didn’t take anything from the apartment either.
a regular joe who wanted to do this would’ve just shot him.
The fact that there were screams loud enough to concern neighbors could suggest it wasn't quite so professional. If it were a professional hitman, why not use something guaranteed to instantly and silently incapacitate him, first, then focus on cleaning up the evidence? Or did they try to, but it didn't go as planned?
One possible explanation would be that the goal was to send a message (to him before his death, and others after it), not to try to make him disappear without a trace. Though, either way, it doesn't necessarily mean it was a professional hitman.