The way I see it, it could come down to one of four, Eric, Chase, Steve, or Wilde.
Chase falling since it would appear that he's been set up.
Wilde jumps up and attacks Eric, they fight and one falls over the cliff.
Steve attacks Eric and the same thing happens.
There's also the possibility that the white van that followed them was some was Lump and/or the former road manager with a long range rifle and shot one of the guys that was still on the ground, or climbing up the rock face.
Jan