I'm more bothered by this than I should be, but there really is no logic in thinking EA waited 2, 3, 4 or more days before disposing of Christina. I mean, he lived with his mom and dad, with his brother home that particular weekend too. Where would he have put her for those days?
I don't think he had it in his mind that he was going to attack her when they walked to the parking garage. I think he heard her frustration with HF and seized an opportunity, thinking maybe he could get lucky because she was mad at HF. Then she fought back and he hurt her/killed her. He wouldn't have had a clue what to do at that point, he would have been panicking and wanting to be done with the situation immediately.
This wasn't a psycho serial killer type of situation where he wanted to savor every moment and relive it. This was an omg wtf have I done moment. If he'd been planning things out, he wouldn't have had damage on his car, he wouldn't have been injured, he would have showed up to work as casual as could be, right on time, while CM was locked in a storage facility or a basement of some abandoned house that he had scoped out.