I must agree with you on some points. Lupin did act atrociously in DH. But I also think it was necessary for the book. Not only that, but I can understand why he was thinking what he was thinking. Not that I condone that behavior at all, but I do understand it. And, conversely, I understand Harry's reaction.

Lupin's way of thinking is, I've condemned the woman I love to such a base way of existence as to lower herself to living with a werewolf, who isn't good enough to spit-shine her shoes. To make matters worse, I go and get her pregnant, further risking her life (what if the baby turns out like me--a werewolf?) and the baby's. What kind of monster am I, that I am able to do this without thinking of the consequences until it's already said and done?

So Lupin tries to rectify the situation by offering his services to Harry. He truly (and very misguidedly) believes Tonks and Teddy would be better off without a poor excuse for a man like him to make their lives miserable.

And, on the other side, we have Harry's point of view. Harry, the lovable orphan who has spent the majority of his life not ever knowing what his parents were like, has suddenly found himself pitted against his father's best friend in a situation he knows his father would never in a million years approve of. And Harry feels it is his duty to act in his father's stead. Plus, he feels almost as if it were he who is being abandoned yet again. It is completely selfish of Lupin to abandon his family at the most crucial time in their lives--Tonks needs her husband to help her plan this baby! And to help her raise him! And Lupin wants to rush off! Harry can't help but feel that Lupin doesn't deserve to have a family if he is willing to abandon them. Harry's parents didn't have that choice. And Lupin is choosing wrongly.