I remember hearing about this horrible crime. You wonder what on earth his motive could have been....what was he thinking in order to do such a thing?
No doubt there was a pharmaceutical component.
In reading the linked Forbes articles, it appears that Taylor Dawn Paschal-Placker's older sister was (or had been) dating the perp. While the articles don't specifically say so, the older sister may have been the "former girlfriend" he was dating at the same time he was dating Ashley Taylor, prior to killing Ashley.
Could be the prior girlfriend was trying to break off their relationship. Likely the perp happened upon the girls walking down the road, and took his rage out on his ex-girlfriend's little sister and the friend who just happened to be walking with her at the time. The little sister was shot several times in the face, while the other girl was shot several times all over her body.
Some people are born deficient. They lack the ability to feel remorse or sympathy. They are called sociopaths.
Not all sociopaths become murderers. But some really, really enjoy killing.
This fellow sounds like one of the latter.