From what my wife has been reading... it seems that this was a woman that McNair had been "supporting" (e.g. house, car, cash) on the side from his wife, and the speculation is one of two scenarios: A) The woman was unstable and this was a murder-suicide, or B) Her ex-boyfriend was very jealous of the two of them and killed both of them.
Either way, very sad story and my heart goes out to his family.