I think, in a bubble of just this fight, Randall and Kevin would be pretty equal on the "heinous remark" meter. Where Randall comes off worse is when you look at the sum of the parts of what we've seen in the show.
A few people mentioned Kevin before his growth and maturing, and I personally view his behavior then as a result of the favoritism Randall clearly got. I don't necessarily think Kevin would just resent Randall or would have been particularly attention-seeking and bratty when they were kids without that occurring. For me, it's important to recognize what they *both* went through, how *both* of their lives were affected by Randall's adoption and the subsequent actions and moments related to that.
I'm having trouble trying to get it out succinctly (too verbose for my own good), but I think that, because of Randall's set of issues connected to it, it naturally gets understood and cared about. Rightfully, mind you; not trying to argue what he had to adjust to isn't important whatsoever. Because Kevin's set of issues isn't generally seen as "heavy" as Randall's, it gets forgotten about and ignored; weirdly appropriate, considering what those issues are.
What Randall isn't seeing are all the times Kevin took it on the chin with regards to his part in the family. His *own* issues prevent him from seeing beyond the surface-y success Kevin has, while at the same time passing judgment on it. We've seen him pass judgment on Kevin with no prompting action from Kevin (ex: his snide behavior regarding Kevin's acting workshop) and we've seen him outright dismiss Kevin's perspective and feelings when Kevin was simply trying to be open in order to better himself (like the family therapy session in rehab), with what feels like little acknowledgement for Kevin's support. That goes for when he was young (helping Randall with forging Rebecca's signature on a school note, helping Randall win Beth over) and now (bolting on opening night of his play to help Randall in his breakdown). When you put it all together, it starts becoming more "Kevin said something harsh in a heated moment; Randall was an arrogant jerk who was far too cruel, especially as the wrong party".
For me, the most compelling part was their reads on each other and the situation. Randall was way off base, while Kevin was spot on. That jives with who they are; Randall is intellectually intelligent but socially tone deaf, Kevin isn't as book smart as Randall, but is far more socially adept. And Randall doesn't see that, because he believes his academic intelligence gives him the ability to understand anything, failing to realize that it doesn't work that way at all. Kevin's dumber than him, so Kevin's read on him *can't* be right.
It's all the history and layers that I think wind up making Randall seem way worse overall.