I'm not surprised. Don is a very complex character but you seem to see only the surfacy, obviously assholish and contradictory stuff that Don does. Weiner has, over the last 6 seasons, shown us what, and from where Don came, and why he is who he is. There is much more to the character than the things you choose to focus on.
I know there's more to the character than what's on the surface. I see what Weiner has chosen to present as the traumatic events that molded Don, but Don's stinging cruelty at several points during the series have often surpassed what might be considered expected due to the abuse Don suffered. There are times the character's behavior has been completely beyond the pale. Shocking, even. Hamm really nailed it. Don completely rejecting his brother was one of those moments. His treatment of Betty at times really tiptoed that line.
I don't know if its Hamm's portrayal, or the writing, but I've always had a hard time sympathizing with the character. Pete's done equally horrible things, but there's something about the pathetic-ness of Pete and his lack of social graces that makes me feel a level of sympathy even though he deserves a thousand smacks for his behavior. Could be the actor. Kartheiser's never gotten enough kuds, IMHO.