I agree. He may well have carried one all of the time in the glove compartment or on him, I would think more likely in the glove compartment--if it was on him, I definitely think it was a plan. If he had to go to the vehicle to retrieve it, he also had time to think twice. However, some thoughts depend on where she was killed. If it was in his vehicle that he shot her eight times, and we don't know, that is one thing. If it was, his truck is a total and obvious crime scene with eight shots. If it was outside and he had to go retrieve it, that changes my thoughts as well.
I lean towards he planned on killing her when they left town if she would not agree to what he likely wanted.
We have a lot unknown.
All jmo.