A mentally handicapped child was bullied for his childhood and adolescence, etc
You could say it is the bully's fault for bullying the child.
You could say that it is the father's fault for overreacting to the bullies.
You could say that it is the parent's fault for not leaving the area.
You could say that it is the bully's parent's fault for not raising the kids better.
You could say that it is the fault of the school and legal system for not preventing it.
You could say that it is the fault of the neighbours for standing by and letting their neighbour be harassed in such a way.
You could say that it is the fault of media the bullies were exposed to that encouraged them to bully others, and not to empathise.
With most situations, you can blame almost anyone, and humans like to blame other humans. Whenever something unfortunate happens to someone, they tend to find some way of shifting the blame onto some figure they already dislike. This can take many forms; for example, no matter who I ask, they say that the media is biased against their political views.
There is no point in saying that someone is to blame or not. Not only is blame a very slippery and malleable concept, but it can very easily lead to hating the person you deem responsible, which is never a good thing. Two wrongs do not make a right.