In my view the world's deepest thinker on bullying is the developmental psychologist Gordon Neufeld (www.gordonneufeld.com). He sees bullying not as a behavioral or moral problem, but as one arising from disturbed attachments--of both bully and victim. His DVD on the subject may be purchased here: http://neufeldinstitute.org/category/dvds/ A free YouTube lecture may be seen here: https://www.youtube.com/watch?v=q7mznfMI1T4
(disclosure: I'm the co-author with Gordon of the book Hold On To Your Kids: Why Parents Need to Matter More Than Peers, which contains a chapter on bullying)