It is childhood, unfortunately. Mockery is almost second nature to them, because at a certain age that is how children act to separate themselves from others, and to define their very own personality ("I am not like you, because you are old but I am young"; "I am not like you, because you sit still while I am active playing football"; "I am not like you, because you are stupid but I have good grades"). This carries on into adulthood, of course (although it is more subtle then), but when you are that young there is - for some reason - a compulsion to set yourself apart, and define yourself as "the superior person", by spouting abuse. And with it comes the group mentality, the thought that it is Them against Us, and that you need to join with whoever is most like you. Or rather, to join with the attacking side.
I do feel sorry for this poor woman - it must have been extremely hard on her when her husband died, and very possibly more so when her son committed suicide, but to have it all brought upon you again, by jeering faces, with the added message "nobody loves you"; that must also have been very tough. It is something I do not wish upon anyone.
Their reason for posting the video may be clouded, but again, these are children - and they do not have the same mentality that we do. They thought, since they were supported by their peers and had their laughter to back them up, that everyone else would do the same. It may not have been conscious judgement, and it most likely was not, but such is the curse of our race that we get caught up in immediate detail and never look at the big picture, or even any part of the picture beyond ourselves. And I am reasonably certain that quite a lot of grown-up people have had ideas similar to this, but have been held back not by compassion but by experience of the world.
Humanity is, invariably, dark.