Sunday will be the test. Will regular News of the World buyers pick up another paper instead? Will advertisers want to remove their brands from a toxic publication? Or will millions of us - remember, it is in the millions - just carry on regardless?

The allegation that an investigator, paid by the News of the World, hacked into the phone messages of murdered schoolgirl Milly Dowler, is truly shocking. The further allegation, that messages were deleted to make room for more - giving a family false hope that their daughter might return home safely - is, if true, a thoroughly callous and despicable act.

There is a sense that a line has been crossed this time. The phone-hacking saga was barely of interest beyond the media bubble when it involved politicians, or even celebrities; but this new revelation is truly sickening - sickening for anyone who considers themselves to be a journalist or who cares about the ethics of their profession; and sickening for us as punters, as people who buy newspapers and care about what they produce.

There are times when breaking the law to get a story is justified, and there are times when some behaviour, even if it doesn't break the law, cannot be justified in the context of getting a story. This case, it would appear, is the latter. A police investigation was ongoing, and may potentially have been hampered by the actions of the News of the World's investigator. So who knew what was going on, and who is to blame?

As it stands, we are told that no-one knew that this had happened. And there is no reason to suppose this is not entirely true. But even if this was the 'one bad apple' who took things too far, an investigator who had gone rogue in the quest for new stories, completely outside of the knowledge of every single employee of the News of the World, I do not think that means that no-one there can be held responsible for his actions.

Who knew? We will be asked time and time again. Perhaps a better question is 'Who should have known?' or 'Why was a culture allowed to develop in which this kind of behaviour was seen as justifiable or acceptable?' The editor in charge of the News of the World at the time was Rebekah Brooks, now the chief executive at News International, who says she is as shocked and surprised as anyone.

Those of us who are appalled and dismayed by this latest story must be careful to act responsibly with our understandable anger. If we do not, we run the risk of being no better than the mobs who wrongly targeted innocent citizens after the News of the World released paedophiles' details back in the early 2000s.

That 'naming and shaming' was part of a campaign for 'Sarah's Law', where the newspaper placed itself on the side of victims and their families, demanding justice for those affected by crime. That it should have happened at a time when one family were apparently being given false hope that their daughter was safe, just so that someone working on behalf of the same newspaper could read more harrowing and intensely private messages left from concerned friends and relatives of a missing teenager, puts everything in a new focus. It is a messy, horrible and deeply saddening story.

And it's that humanity, the horrendous ordeal of the Dowler family, which must be kept in mind at all times when discussing this episode. It is at the heart of why this story matters, and it is at the heart of why this does not become a gleeful witch-hunt.

Instead, let the facts speak for themselves. If readers are outraged by the latest allegations, the easiest course of action they can take is to stop buying the paper. We will have to see whether that happens or not, starting this Sunday.