There is a lot of violence that is not recommended for younger teens. The story starts with Zits, who is an orphan, entering a bank and shooting people and then turning the gun on himself. The time-traveling as a character-development tool works. Zits is able to grow by traveling through time and becoming different characters: an FBI agent, a mute Indian, an pilot instructor, and even his own father.
Zits (and the readers) see how the legacy of violence has evolved, and recognizes his part in this chain. For a less violent book on self-identity, read Alexie's Absolutely True Story of a Part-Time Indian.