Like most spiteful, hurtful and unkind behaviour, it stems from deep unhappiness. I imagine these sorts of people feel unaccepted by the rest of society, and possibly isolated and unwanted. The internet gives them an anonymous way of 'getting their own back'. I feel very sorry for them actually. It must be very painful to be so miserable that one feels the need to attack and hurt with words.
I saw several children like this in my teaching days. The ones who bashed weaklings in the playground, or scribbled on someone's nice piece of writing or called other children unkind names. They were always what I thought of as 'The Unloved'. One could punish them of course, but I found a better way was to praise them and show them affection, give them responsibility and respect. It often brought them round.
I wonder if there's a similar way to deal with trolls? Post something nice about them, or say something kind perhaps?