I have two thoughts floating in my head, both just speculations/my opinion.
1. He might actually be very good at this, back in 2009 he was "just starting out", and has possibly gotten better at what he does. He has seemed to have hidden Sierra very well, and from what I have read so far only skin cells on her clothing (nothing super obvious like sperm or blood). And all his friends and family members have spoken out in disbelief, no signs of this side of him, if they are being honest. I have a hard time believing that he has been violent towards women in the past (his then gf, now wife), and linked to at least one 2009 assault and has sat around for the past few years doing nothing. I wouldn't be surprised if he ends up being linked to, or confessing to one or more victims. The bag does puzzle me though, why not just destroy it?
2. My other thought is, maybe he did not plan to murder her. Maybe he was just planning a rape/release, but Sierra saw him (if he was initially masked or behind her?), or had an asthma attack or he got a little too rough. To me, this might explain the folding of the clothes, and hiding her things in a safe location. Maybe he was feeling guilt and remorse and was trying to "make amends" in a way by being respectful of her things? Although if he was remorseful at all he would probably fess up, not plead the fifth.