'If only's' aren't always logical but they are a natural response by loved ones to a tragedy like this. If Helen put her stamp of approval on IS and intended to marry him, it was very difficult for anyone to rain on her parade and express reservations about him and his motives. Hindsight's a wonderful thing and no one could have known even a fraction of his true nature and - if he had killed Diane - his history. But certainly the speed with which they got together may have rung alarm bells with some of her nearest and dearest and if they were aware of her changing her will in his favour before they were even married, it may have provided a clue that he was manipulating her behind the scenes.
One of the big themes of Planet Grief was personal safety for widows/widowers embarking on the dating scene and Helen specifically mentioned keeping details of your finances guarded. Yet within only a year of meeting IS, she asked him to witness her will which revealed she was worth well over £3 million! Her defences were entirely down with this man. Then he set about persuading her to change the will to favour him, with manipulative pleas about how worried he was about the future security of himself and his 'boys', and his murder plans proceeded apace.
The fact that so many of her friendships lapsed after he came on the scene (including it seems with her dear brother John, who stated he only visited her twice at the Royston house) is one of the biggest red flags of all.