Almost all of Pratchett's greatest characters are highly flawed, morally complex and anti-heroic. This is the main point. This premise includes everyone from Cohen the barbarian, through Vimes, Rincewind, Susan, all the witches, Moist Von Lipwig, all the way to DEATH.

That's one of the main reasons that Terry's work comprehensively bridges the genre gap between "children's books" and "modern philosophy".