I think RH is 100% responsible for his son's death, but I do not think it was premeditated.
My hunch (reserving the right to change my mind) is that he knew Cooper was in the car at HD parking lot. He left him there "just for short time" while he made an appearance at work, and he fully intended to return within a safe period and bring Cooper to daycare. I think he has done something similar in the past either at work or elsewhere.
I came to this conclusion by considering RH's personality and behavior. He's lazy, takes risks, lives in the moment, is selfish and immature, likes to be with people, doesn't have a good work ethic or ambition, avoids problems with distractions rather than taking action to solve problems. I do not think it crossed his mind to murder his child. If he did want out of the marriage, he would do so (and was doing so) in a passive-agressive way by being a selfish jerk whose wife either put up with him or left.
I also don't see where he makes any goals and then actually does anything to achieve them. He wants to be VP at HD, but shows up late, leaves early and sexts on the job. He wants a start-up business but he and his buddies go to the afternoon movies rather than busting butt to get the company off the ground. He wants a new house, but spends money rather than saves it.
I really don't think he has it him to want to murder his child or to actually make a plan and follow it.
What I do think is that he got distracted by his dingaling at work and forgot his child was in the car. I think he knew either before lunch or at lunch, but - in his problem-avoiding way - neglected to do anything about it. I think his "discovery" was fake and when he walked away to talk on the phone, it was once again his passive problem avoiding characteristic showing. The guy can't face up to any responsibility.
But he didn't plan it this way. He did, however, let it happen. It was his inaction rather than his action that caused the death of his child.
JMO, all speculation and hunches.
