Don is such an unfathomable cretin. He acted the same way towards Betty when Roger came to the house for dinner and flirter with her all night. He made snide accusations to Betty all week when he's the one fucking around, per usual.
I've always cringed at what a jerk he can be, but every once in a while he outdoes himself. He's no better than Pete. He's just in a prettier, slicker package. Hell, not even Pete would pull the jealousy routine with Trudy when he doesn't love her.
It's awesome that Peggy stole the account away from him. Poor Joan, but it was inevitable that the way she got partnership would come back to haunt her. The shame of it is she deserves it for her intelligence, but she'll always be accused of taking the easy way when it's been anything but for her.