Lupin had a low self esteem. A very low one. I don't think his actions towards his wife and child were right, but he didn't think himself worthy enough to be able to care for his wife and child. He thought that they would be better off without him, because he was scared of hurting either one of them. There's also the issue of Teddy having the same condition as him, and he would blame himself for that.

I do agree that his actions made me respect him slightly less, but his past experiences and condition made me understand why he took the actions that he did.