Randall did have the benefit of knowing the argument was coming while Kevin did not. Randall knew that Kevin would be upset if he figured out that Rebecca was coerced by Randall, so he could practice his justifications and comebacks to Kevin.
Plus, as we saw last week during his fantasy alt-realities, he does actually think all those things about Kevin. Randall probably feels great that he finally got to tell Kevin what a loser son, loser person and loser actor he is.
I just can't get over how he keeps dissing of the acting. I can get that his view of Kevin is biased, but how can he miss that Kevin is a reasonably successful actor who is being courted by well-respected directors? Kevin first created a television character that anchored a successful show for years and then flipped from being stereotyped as a himbo to getting meaty dramatic roles - that's not an easy transition to make in the time period given. And Kevin was the architect of that transition by going to New York and producing a play which could demonstrated his dramatic chops. Sure, there is still a huge element of luck, but how can Randall not see any of that?
Other than allowing Kevin to crash in his basement, has Randall ever really been there and supportive to Kevin? (I guess he went to rehab, but he wasn't really supportive).