BBM
What if this crime was not only not targeted, but not even planned at all...maybe it just unfolded as a random crime of opportunity? Maybe BG (a mentally unstable guy with a tramatic background and a quick fuse) had a really stressful morning (maybe an arguement with his parents, or got dumped by his girlfriend, or lost his job etc...) so decided to walk the trails to blow off steam. The bridge might be a place he went in his youth, and he might find comfort in being outdoors. He bundles up in a hoodie and jacket and maybe a hat and scarf, because its winter. He is not trying to disguise himself, just stay warm. He parks somewhere familiar at the CPS building (as locals often do) or cemetary. He is not worried about his car being spotted by anyone, as he has no criminal plans. He is carrying a gun and/or knife, not because he intends to harm anyone but because it is common to do so when hiking in rural Indiana. There is no 'kill kit'. At some point during his hike he spots Libby and Abby. For reasons unknown he is triggered by them and starts having dark impulses. Without even thinking he starts following them towards the bridge, creeping them. He enjoys following them, like a predator stalking its prey. It makes him feel powerful and in control and he forgets about his troubles. When he catches up with the girls at the end of the bridge there is nowhere else for him, or the girls, to go. He senses they are afraid, which feeds his ego. He can see that noone else is around. He impulsively pulls his weapon on the girls "Guys". They freeze out of fear. He then orders them "down the hill" to whisk them out of sight. He has no plan or contingency for if one/both run, but he is lucky they comply. At this point he can't believe what he has just done. He didn't mean for this to happen but knows there is no turning back now. He is in a panic but since he knows the trails well enough he is able to find a secluded spot to finalize his attack on the girls. He murders them quickly (to silence the witnesses) but possibly brutally and exits the area as fast as possible.
This is all JMO