People have always picked on those who are in some way different - people with red hair, with glasses or who are obese will all have been subject to unpleasantness of one kind or another. I think that there are those who just want to feel that someone is lower down the pecking order than they are, and look for any excuse. Some sniff out victims for no obvious reason other than that they have taken against them for some reason - a perceived slight ('disrespect') or because they don't like their shoes or something equally superficial. I don't think they are necessarily 'hate filled' - it is more complicated than that.
The term 'bullies' is so subjective, and many people who call others bullies do do in order to bully the accused. We see that on here all the time, don't we - it's an effective way of shutting people down when they are saying things the accuser doesn't want to hear. Murder goes way beyond any of that, surely? It may have some of the same causes (needing a scapegoat, personal dislike etc), but I think something different is going on when someone is killed without there being an obvious 'reason' (eg a fight or a robbery). Is it a mental health thing, or were drugs/drink involved, for instance?
Obviously this is all way into the realms of speculation. Maybe we will never know, and maybe the police will be able to reveal the reasons (although this all goes way beyond 'reason') after the case has come to court.