You're just showing your ignorance. The Nazis killed the majority of their victims as soon as they arrived in the camp. They kept 20%-30%, mostly men who could work, to be used as slaves for their military industry, and they eliminated everyone else. They developed gassing especially in order to speed up the killing and make it more economical.
It was a genocide because they tried to eliminate a specific ethnic group, for no other reason than its ethnicity.
You should go learn more about the subject, if it interests you so much.
