Yup, the answer to both of these "shut your mouth and let it happen". Complaining about your friends partners always leads to losing the friendship and never to them ditching the partner, no matter how horrible that person is (or isn't!).
After reading Fatal Vision, Jeffrey MacDonald seems like an awful human being, which he might well be. I think it's completely possible he killed his family. However, the standard of conviction has to be without a reasonable doubt. And I certainly do not get that from the known facts. I would rather a murderer go free due to reasonable doubt than an innocent man serve three life terms in prison.