Heres the timeline of events as I imagine them to be.
I find it possible that AS was involved in the 2010 assault at ketner lake, although LE has not linked them. We know it is fact that he was involved in the more recent ketner lake rape/assault attempt due to dna link via LE.
It is fact that he took Jessia, sexually assaulted her and abused her remains.
I posit that he tried in the 2010 attempt and was thwarted. He manufactured some chemical to facilitate gaining control of the woman more recently at ketner lake but was still unsucessful.
He begins channelling that rage and aggression (if he wasn't already into animal torture and killing.) This feeds his sick urges for a while and he enrolls in the forensics course and explores a carreer in mortician sciences. I believe this is when he realises that simply raping someone will not be enough to satisfy. He needs to inflict pain and violate more completely, more finally than rape will allow.
Enter Jessica, alone, small of build, weak and vulnerable, much like the animals he has been "playing" with of late.
This of course is simply my own speculation about AS progression as a killer. I believe wholeheartedly, with every fiber of my being that AS was just beginning. He would have been a serial killer unapprehended. And as he himself reportedly stated to LE killing Jessica did not give him the joy he expected it to. His horrendous acts would have continued, excelerated.
I feel just as strongly that Jessica was his first human kill.