
Just for the sake of discussion, only, and I am in no way excusing the behavior of our Dr. McDreamy, but if you look at the blog from the writer/creator of GA on "Grey Matters" she explains that one of the reasons she chose to go to flashbacks on the season opener was to REMIND folks that it was Addison who first cheated on DMD and with his BEST FRIEND, which he caught in the action! She wants to remind us that the situation almost destroyed him and by the time we are introduced to the character, (meeting Meredith in a bar, who also has her "issues"

he is in a very sad, vulnerable place. It is later that Addison follows him to Seattle so I think she (the writer) wanted us to suspend judgement and try to understand where the character was coming from before all this confusion of Addison returning happened. Does that make sense?
If not, read it in her words on her blog - it's very interesting to hear all the symbolism and dimension of these characters!