This isn't Melvin Burgess' best book. Lines like "the vanguard at the front" seem unnecessary. Either assume your audience know what vangaurd means or leave it out. Don't go; ooh I know a long word but you're too thick to understand it.
The characters didn't appeal to me, especially the main character Adam. He's just a thoughtless, selfish teenager. Maybe this is the sort of book that's supposed to get teen boys reading. I hope there are teenage boys out there more likeable than Adam. I know didactic literature isn't the way forward but surely your characters should at least be likeable?
Then of course there's the psychotic villain who's stopped taking his meds. Of course in the real world mentally ill people are more likely to be the victim of crime. But hey let's not let facts stop us. I think it would have been much more interesting if the protagonist struggled with mental illness and that's why he decided to take Death. The characters in Junk do foolish things but I felt I could understand them better.