The character of Willoughby serves to underline Austen's point about love and marriage. She depicts him as a charming and very handsome young man, with all sorts of assets that are particularly appealing to young, inexperienced (as they should be), impressionable women. He enchants Marianne nearly at first sight. Naive as she is, she falls in love immediately, and finds all sorts of virtues in his character. Actually, she sees in him everything she secretly wants to see in a potential husband. Austen depicts him as seen through Marianne's eyes, both at the beginning, and later in the novel.
But Austen doesn't seem to condemn Willoughby for his superficiality and immoral behavior. She gives him another chance, even if he has to spend his life in an unsatisfying marriage, as if to atone for his past sins toward Marianne. Austen always keeps the rational tone, letting the characters' actions speak for themselves.
However, her depiction of Colonel Brandon puts him in favor over Willoughby. He is a stable, older man, who isn't nearly as charming or handsome, but obviously has inner values, moral, intellectual, and emotional. The fact that Marianne eventually marries this worthy man, while Willoughby is left in his unfulfilling marriage, is Austen's hidden verdict. Everything has its own time, the novel says; the youth carries a certain level of thoughtlessness (such as both Marianne and Willoughby show us), but there comes a time that calls for mature decisions, and employs sense over sensibility. Marriage should not be a shallow economic union of two people, in her opinion. It reflects all the values of two individuals, their families, their social class, and the society they live in.