What is so great about Willoughby is that he is a many-sided character, not just a plain villain but a real person. I think he explains himself very well, but as Rachel G says, it doesn't have the effect he hopes for. He lacks empathy and he is also very prone to need admiration and others good opinion and so he is unable to really understand how his deeds affect other people. He hasn't changed (yet, if ever) for he has no empathy for his wife and no mature understanding how to make their life the best possible.

I still think he was for awhile sincere in his affection for Marianne, as sincere he is able to be. It maybe isn't much and it maybe lasted only for a day or two, but that is the best he can do. We don't know much of Willoughby's past, but as a clinical psychologist I meet this kind of persons and there are reason why they become that way. That it not to say they aren't responsible of their actions. But it is a long way to maturity, as Rachel G says.

So, I feel sort of pity and sense of waste, like Elinor did. I think he did explain himself and in explaining he revealed his psychology. What a great character! What a great writer JA was and what an understanding of human psychology! I wish I had such, too!