My own crazy, wild-arsed guess on what happened is that he had access to Kegan Kline's "catfishing" account and arranged a meeting with one of the girls. When the girls saw him they probably thought something was up and he may have said or implied as part of the whole "Down the hill" statement the person they WERE meant to meet was... Well down the hill.
I don't think we have to guess really about him having access to that "catfishing" account, or at least being partnered with Kline in this whole thing. That "catfish" account was in contact with one of the girls that morning, so it's a good bet the girls thought they were going to meet this young man on the fake account.I think you are right, when they saw this middle aged guy approaching them, they knew it was bad which is why the one girl turned her phone on and recorded him. Something I learned, the trail basically ends on the side of the bridge the girls were on. It's there, but badly overgrown and would have been hard to navigate. They were pretty well trapped.
It's also know Kline's phone pinged at a nearby location that same morning. It's thought he was waiting to pick this Allen guy up after the murders.
What is known is that just about two months ago the Indiana State Police sought to have Kline removed from the county jail, which was allowed, and they took him to an old airport that has a jail facility on it. It's very close to a spot on the Wabash river where police spent several weeks searching the river. Guess is, Kline disposed of something in the river, gun, etc, that Allen had used in the murder. That's why they had him moved to a different location, so it would be easy for them to carry him to the spot on the river and point them to where he disposed of any items. After that search concluded was when they arrested Allen so it seems connected.
Kline also deleted a bunch of stuff from one of his phones, a Snapchat app and another app called MeetUp or something like that. I've heard speculation that Kline's father is involved. He and this Allen guy are around the same age and might have known each other, but it's only speculation.
It's seems to me that if Kline is involved in any way he would be charged. I suppose he could be charged yet, but maybe police think the charges he is already facing will be enough to keep him locked up for a very long time and they can always get to him later. They might need to see what info Allen gives them about Kline.