Well, unlike his friends and family and coworkers, we know a great many of RH's dirty little secrets, and imo what's becoming sillier by the day is the State's argument that his dirty little secrets made Ross capable of murdering Cooper.
I wouldn't buy that as an argument. However, my feel is that the state is trying to show that none of the people thought RH was capable of cheating to the extent he did, yet there is solid evidence that RH shocked almost all of them with the extent of his infidelity. The state is saying the same thing could apply to poor Cooper; no one thought he was capable of harming him, but maybe he was.
In terms of motive, I'm not sure. To be fair, I have only been reading recaps and here so I may not be totally informed, but I didn't think they were saying that the cheating led directly to the death of Cooper; instead, that both the cheating and death were ways where RH tried to escape his responsibilities to his family. Again, I could have missed more about the motive.