but why drag the body up to the roof of the McD’s across the street?
No one said they were smart. Murderous ferals yes but smart no.
It could go undiscovered there for a long time.
The perps were unlucky this time because someone found it right away. I bet if the guy just disappeared and no corpse was found for a few months they would have gotten away with it.
Witness memories fade, hard to establish exact time of death, etc. Even if the cops could place the woman in his apartment on a certain date, it would be very hard to establish that was the date of death, and she could just say; "He was fine when I left."