I think it is important to understand that great literary works, such as O'Neill's play, hardly ever make an explicit endorsement or refutal of the characters, but rather lay the bare facts of their lives before you and then force you to make a choice/interpretation. Mary Tyrone is a very complex character, a woman addicted to morphine and in a permanent state of retreat from reality. In that respect, we are tempted to dismiss her as just a junkie who does not have any moral/ethic stature. And yet, O'Neill carefully tells all the circumstances in her life that led her to her present condition. As an adolescent, she cherished the dream of becoming a pianist, and even thought of becoming a nun. But all her dreams she sacrificed for an actor, James Tyrone, with whom she fell madly in love, and married him despite her family's strong opposition. Her subsequent life was characterized by the lack of a permanent home, an itinerant experience that took her from one cheap hotel to another, always following her husband and his acting career. She had great difficulty in delivering her son Edmund and her husband, a very stingy man, sought the assistance of a whack doctor who simply administered her painkillers, turning Mary into an addict, etc. As a conclusion, the play does not take sides, at least not explicitly, and limits itself to reconstruct the lives of the different members of the Tyrone family, all weak and detestable in many senses, but all with enough reasons to be the way they are. Don't forget that this play was not released until after the author had died, for it lays bare the history of his own family, the O'Neills.