I think it is inconceivable that the editor of a 'newspaper' didn't ask what the source was for all these celebrity gossip stories. There are only two possibilities:
First, as the editor of a national 'newspaper', he was willing to publish unsubstantiated stories about celebrities and members of the royal family without checking their veracity; second, he was aware that the information was obtained illegally and published the stories regardless. It really doesn't matter which scenario is true because it demonstrates that he was either incompetent or willing to indulge in illegal activities.
I'm actually amazed that it is only the current editor – Edmondson – that is being questioned about this. From what I've read, Coulson (who happens to be our PM's right-hand-man) should also be under investigation for exactly the same accusations.