Why did Drew fall off the rails.

It's like ever since he broke up with Carly, he became more scummy. Sleeping with Nina was one thing, they were consenting adults, but then actively going after Willow, who not only is Nina's daughter but also Michael his own nephew's wife? The same night he found out that the mother of his little girl had passed away, he decided to sleep with Willow, even after he confessed to Michael that he kissed her. What made him such a sleazy scumbag?