I think they didn't want the murder to go unnoticed. I think they definitely wanted their names out there and notoriety hence the burning up of vehicles. Dare I say, maybe that's why they chose a young, attractive couple as they knew the media would jump on that and be all over that (which they shallowly and sadly do).
this would imply that they saw CD and LF very clearly somewhere - earlier in the day or earlier in the trip and selected them, not that they just came upon them or the van in the semi-darkness and attacked because they thought the van was empty or isolated