At first, Mr. Morel doesn't seem like such a bad guy. In fact, we totally get why Gertrude falls in love with him. He's got a serious zest for life. He's into dancing and singing and generally having a good time.
You can see all these qualities emerge in his first meeting with Gertrude: "He came and bowed above her. A warmth radiated through her as if she had drunk wine" (1.83). At this point, we might all be thinking, "Thank goodness the guy isn't much of a drinker."
After all, the narrator tells us that Walter "had signed the pledge, and wore the blue ribbon of a teetotaler" (1.107), which means that he never touches a drop of alcohol. But Walter decides quickly after he gets married that he actually is a drinker. Now working up to your neck in black coal all day might make any of us want a drink now and then, but Walter quickly forgets how to say no to alcohol, ever.
Once he starts spending his family's money on booze, our sympathy—and his family's sympathy—for him starts to crumble. When he actually starts getting physically violent with his wife and kids, we just can't put up with this guy anymore.
You know, Walter isn't as dumb as he might seem. He knows that his children have all sided with their mother, and doesn't like feeling unwelcome when he comes home. But instead of giving up alcohol—the thing standing between him and them—he just gets drunker. And angrier.
All of his sins might actually be forgivable, if he were ever willing to apologize. Okay, not the being violent to his women or children part. Still, his failure to ever discuss his problems openly eventually destroys all lines of connection with his family, until "Conversation [becomes] impossible between the father and any other member of the family. He [is] an outsider" (4.77).
Now and then, Walter and Gertrude have some tender moments nonetheless. Like when "He saw again the passion she had had for him. It blazed upon her for a moment" (8.297). But as the book unfolds, these moments disappear, and Walter becomes a sort of ghostly presence in the Morel household.
He doesn't really know how to react when the family involves him in any sort of decision or celebration. As the narrator informs us:
He was shy, rather scared, and humble. Yet again he felt his old glow. And then immediately he felt the ruin he had made during these years. (8.297)
Sadly, Walter knows that he's done too much bad stuff to be forgiven, so he continues to come "home" knowing that there's no real place for him there. His only comfort comes from drinking with his mining buddies. Which doesn't add up to much of a life, when you think about it.
The extent of Morel's alienation from his family is best captured in his reaction to his wife's illness and death. When he feels that his wife is about to die, Morel goes "obediently out of the room" (14.310), just after he "look[s] at his son, helplessly, and in horror" (14.305). Yowza.
Frankly, the man is afraid to be around his wife in her time of need, because he knows that she won't forgive him. And he knows that he doesn't have the strength to apologize and ask her for forgiveness. So he just remains really, really awkward around her, and lets Paul take care of everything when it comes to funeral arrangements.
Walter Morel is a sad dude, all right. But there are pretty clear limits to any sympathy we might have for him. You understand, don't you?