I agree that the motive for the sequence of events is confusing.
To me, a van parked by the side of a highway, especially a highway used by transport trucks, means it's broken down or run out of gas. So there's no way to steal it, plus it was an obviously a crappy van anyway.
If the goal was to see if there was anything to steal, why would they bring a gun along? If someone was inside, you could just run back to your vehicle and drive off. Or maybe panic, injure one person andcrun away. Shooting two people dead is way too much violence if your goal is to steal some money and continue on with your life. Killing two people doesn't just happen accidentally, it is a very deliberate, massive transgression against our social values.
I think they wanted to see what it was like to kill someone. The van was in the most remote, unobserved place the suspects had ever been, no one could hear it, no one would see it, no one knew the suspects were there, it had easy access to their vehicle to escape and go so far away no one would ever suspect them.