Context is important as it is on here. There's a difference between attacking for no reason and doing it in response to being attacked oneself.
On twitter if you call me a nasty word to my twitter face I can block you or otherwise ignore you or reply to your tweet abusing me by abusing you back ... and twitter being twitter anyone else who got out of the wrong side of the bed that morning can join in to the melee should they be into that sort of thing.
What though, if I am an onlooker who happens to know you and everything about you and your family ... where you work, your home address, your children's school etc etc etc ... who decides to stalk you ... who decides to troll you ... who organises a small group to join in the fun.
How would you feel if tweets started appearing about your visit to the supermarket and what you were wearing?
How would you feel if a leaflet campaign featuring you targeted your neighbours?
How would you feel if details of the times of your children's route to school was tweeted to all and sundry.
How would you feel if while you were dining with your children a picture of you all was posted on line giving precise details of your whereabouts.
How would you deal with the general everyday tweets expressing hatred for you and your family over an eleven year period.
According to your posts all of the above and more is legal ... oh well.
However ... the point is that Brenda Leyland was in my opinion doing precisely what you have said in your post and was "attacking for no reason and doing it in response to being attacked oneself".
The objects of her vitriol weren't tweeting hatred to her ... it was all unwarranted by any stretch of the imagination and all very one-sided.