You summed it up well OldSteve. Criminal psychology/criminal profiling is a deep and complex science, based on the cursory research I've done. (Because the human mind, chemistry, and emotional makeup is so complex!) There is a whole study and science just around the profile of stranglers, and then subcategories of stranglers that include sexual attack, other forms of assault. It's also interesting how mental illness potentially threads in to the "why" of the motivation for these killings. Deep stuff, indeed. I found articles and interviews from Pete Klismet and Candace DeLong quite interesting, as well as some excerpts from college courses in criminal psychology; all on Google. Books from the folks I mentioned and several other experts can easily be purchased on Amazon.
So this being said, you honed in on the head scratcher. A pretty young gal is found strangled, but no sexual assault. If sexual assault/sadism was not the motive, then apparently control & punishment are a likely motive. And here's what I'm trying to think through. Coming to her apartment with the potential of detection was daring and bold. Is this person an adrenaline junkie? Do they like the "thrill" of the kill and an equal thrill of the potential of being caught? Or, were they desperate to silence her before she spilled the beans on something?
"As far as the latter speculation, if she knew something "heavy", I really really think her twin would know what that was. Can you imagine twins who have been as close as J&J not sharing something important?"
"If forced to come up with a theory, I would say that Julia knew something so damning that the Perp risked the possibility of detection to silence her. And, I'm going to put it right out there; the fact that the POI has been arrested for child *advertiser censored* seems like the potential "reason" to silence Julia. She might have discovered this fact, and perhaps there is even more related to this charge that will be revealed. "
This, of course, is all armchair speculation. I'm sure LE is on the right trail, and there will be much more to follow.