The doors being locked does scream a premeditated detail, but this guy was able to carry out 4 murders and the two room mates on the first level never were disturbed. So its possible the killer had experienced doors with those type of locks before especially if they’re common in that region or among college dwellings/apartments.
And if it was a serial killer there’s always the most disturbing possibility the psycho is a high IQ repeat offender, that is able to improvise on the go, while feeling no remorse..it’s disgusting to describe someone that cruel and evil as being “savvy” but that might be how they view their own self. Hope they find the bastard and let the parents have him. If the police had an air tight case with dna it’d be great to let the maniac believe he’d get the court room media frenzy and the crazy fanfare that comes with that crap, just to be handed over to the people that would deliver true Justice.