Geez, I knew it was more complicated than I thought. Now we got a guy who's wearing his wife's pajamas cutting her already cut throat!

Maybe she committed suicide when she caught him wearing her things and he was just making sure or something.

I suspect he had very deeply seated relationship problems with women. Wearing her clothing demonstrates clearly that he wanted to be like her (and, by extension, to be part of the Universal Feminine), yet when he found her dead by her own hand he symbolically killed her again -- an obvious attempt to deny his feminine side by "killing" that which symbolized it to him. I would not, however, overlook the possibility that he simply didn't like her taste in nighties and killing her again was his way of showing it.