At the end of the novel only one Character survives. Why this character and what is the author trying to say about war and humanity?

Liesel and Ilsa both survive the bombing...... so does Max. Are you asking a hypothetical question?

At the very end of the novel, the only character we're not certain about is Max. None-the-less..... the "Hitlers and Hubermanns, and Maxes, killers, Dillers, and Steiners…" are all spoken of in the afterlife on Anzak Avenue. Your guess is as good as mine.