Hermann is nihilistic in a paradoxical way. He always claims that there is a higher meaning and a reason for existence but it carries the sinister undertone of life on earth being worthless because of this higher meaning.
Hermann is an ‹bermensch who has developed his own morality from events that might have left others empty, and learns from his own experience and way of thinking.
The author needed a character to be a nihilist, with a heavy cultural backbone, so he made him a Nietzsche follower. Only, the author doesn't care to read anything about Nietzsche, so the character's beliefs vary at every needs of the story. Wildly.
Reconstructed: He finds renewed motivation in life through trying to prove to everyone that "Silly Rabbit, Idealism Is for Kids!". The strong motivation makes him a less pathetic villain than an ordinary Omnicidal Maniac, while he himself, through finding more motivations in life, is gradually transformed into The ‹bermensch- all while holding a grim dark nihilistic philosophy.