What a lot of so-called trolls seem to be is 'obsessives'. And what many of them seem to want is to demonise and 'other' someone so that they can then revel in all kinds of abuse and vitriol against that other with a clear conscience.
I've seen some of Brenda L's tweets, and I looked at the website that Rousette linked to. It doesn't really matter what 'side' of the McCann story people say they are on, these people all want to tear into someone on the internet, and do it again and again. They all show a contempt for their targets and a sense of moral superiority over them.
This is a much wider phenomenon than the McCann case. The Meredith Kirchner murder case drew similar responses. There are and will be others. With heavy irony, Brenda Leyland herself became the latest person about whom it was permissible to say anything.
It is a frightening tendency.