In 'Romanov relations', the letters between Anna Pavlovna of the Netherlands and her brothers, there is the occasional letter from Constantine who sounded actually quite nice. He certainly sympathised with Elena Pavlovna as Ivanushka says, and he was very fond of his second wife and their child, from his letters, and there was never any suggestion that he was horrible to them. However, he was married very young and it sounds as if he could be a bit of a bully, so his relations with his first wife clearly suffered from his immaturity (and, in fairness, hers also). The fact that they didn't have any children probably made it less of a problem to separate. It is interesting that a lot of pressure was put on Elena Pavlovna to stay with the odious Michael, but comparatively little seems to have been put on Julie, at least once Paul died.