The quote acts as foreshadowing to the violence that is present in the book, and it alerts readers to some thematic concepts that the author, Robert Cormier, doesn't sugar coat his story telling. A reader can't help but be hooked by those first three words in book. Not only does the quote immediately grab the attention of the reader, but it immediately forces the reader to ask a bunch of questions. Who is they? Who is him? Murdered? Why are they killing people?
The sentence tells the reader that the book is going to have a major conflict, and people are going to get hurt. And that's exactly what the book is about. It's got psychological violence, physical violence, and loads of verbal violence. All three of those are wound up so tightly in the story that Cormier has basically made Trinity High School one horrible place to be. I'll list some themes present in the book: violence, manipulation, power, peer pressure, victimization, and good vs. evil. In light of those rough and in your face themes, the quote "They murdered him" fits perfectly.