I don't know if this helps explain Pete's wife's 'happiness' before she took her life, but I have read that many people who take their own lives show happiness before they do it. Once they make up their mind to go through with it, they feel relief or euphoria that there will be an end to their pain.
I believe her note was very telling that she just couldn't understand why when she had such a wonderful life with Pete and their boys that she couldn't be happy.
Pete, this isn't your fault. She was in such emotional pain and couldn't understand why she felt like she did. I think she really thought it was the only way to keep from continuing to hurt you and the boys, and therefore herself.
