Because when he said that she was hateful, and that HE was hateful, I think he views himself as the one who should and can execute her (not capture her for trial, etc.) I think he has realized the impact of all the evil that he has done, and that the only way that he can balance the scales is by killing his partner in evil, removing both of them from the world.
I really don't believe at this point that he has any hope of changing her, and I'm damn sure that he's not going back for just another roll in the hay. Unless he murders her after said roll, because surely even Cersei doesn't have the Mountain in her chambers while going at it. At any rate, he has to hope that he has the chance to get close to her, even upright with clothes on, and kill her before the Mountain can intervene.
It's a kamikaze run, a suicide mission.