As a motive for Scott Wallace murdering his wife, Richard Bay introduces the fact that Wallace and his wife took life insurance only a month before her death. If he murdered his wife he doesn't get the insurance money because one cannot benefit from one's crimes. Making it look like a suicide because of this, however, makes no sense because insurance companies don't pay out on suicides, either.
Actually, most policies won't pay only if the suicide occurs in the first two years of the policy.