It would still be murder if someone took him to the mountain and left him to die, even if Dylan had been attacked/consumed by a mountain lion.(or any other animal). The fact that Dylan's bones were found in an area that is not easily accessible from MR's house indicates Dylan did not go there on his own. The terrain is such that traveling uphill is especially tough. While an mountain lion could have dragged Dylan to anywhere up there, it would have been impossible for one to attack him down at MR's house and drag Dylan up the hill that far without leaving evidence of having done so.
****Warning Slightly graphic*****From what I have studied on the terrain maps, Dylan's bones might have even been washed down from further up the creek, and if that has been determined to be true, that would be even more indication that Dylan was left up there deceased or to die. In addition, while this is PURE SPECULATION ON MY PART, there could be some evidence of the bones having been broken before death, again indicating that Dylan was left unable to get down the mountain on his own power.
Also, different animals leave different marks on the bones. If the marks on the bones are only those of scavenger animals (too small to have killed him), then that might rule out death by animal.
I believe that LE has reasons that make them think Dylan was left on that mountain deceased, abandoned, or unable to get down on his own power. I, myself, see no reason that Dylan would have gone up there alone in the first place WITHOUT having contacted his friends first. This just doesn't happen when a teenager is chomping at the bit to get to his friend's house. It's the combination of events, not the single event that makes the determinations on the part of LE.