I think Randall is acting the jerk because he is spiraling into depression and possibly panic and/or manic attacks. The man needs serious therapy. So far, we've seen that he has panic attacks and is an over achiever. As an adult, he quit his job, found his bio-dad, took in his bio-dad, lost his bio-dad, bought the run-down apartment building where bio-dad used to live, ran for office, and moved his family to Philadelphia. In addition, he's had his house broken into and beat the snot out of a purse-jacker. Randall is not okay. He's not in a healthy place. I love and adore Randall, so I hope the therapy works for him and he learns to take a breath every now and then.
Kevin has always been a jerk to Randall, since they were little. Younger Kevin is an absolute snot. Hell, even adult Kevin is an overgrown man-child who can afford to do whatever he wants whenever he wants. He has also exhibits alarming behavior. He sleeps with so many random women, I'm surprised he doesn't have an STD named after him. He has a meltdown on set of his hit TV series, he drives drunk with his young niece in the car (admittedly, he wasn't aware she was there), he turns down jobs left and right, flies to Vietnam on a whim to "find his Dad", tracks down an Uncle (who frankly didn't want to be found), insists on rescuing said Uncle (again, didn't ask to be rescued). Kevin's only redeeming value is that he did take his therapy sessions seriously and is trying to improve.
Truthfully, Kevin is right this one time when it comes to his Rebecca's treatment. Giving Rebecca the one good day before trying to convince her that going away to treatment was the right idea. Randall is right that Kevin hasn't been there the entire time since St. Jack died, taking care of Rebecca. And having Kevin swoop in is frustrating. But Kevin gets a win here because he's actually listening to Rebecca and her needs.
Also, it's fairly obvious that Randall, Kevin & Kate learned to "railroad over others until they acquiesce to your demands" from St. Jack. It and the speechifying are the Big 3's least admirable traits. I cringed watching St. Jack ignore and silence Rebecca all through the New York trip. I know it was part of their generation. But I hated it when my Dad did it to my Mom...I don't like it watching Jack do it to Rebecca.