how the Holocaust can considered a genocide?

Answer 1

Although roughly 14 million people were killed, almost six million of them were from one religious group, picked strictly due to their religion. Many other groups in the remaining 8 million were picked based soley on their race, religion, or lifestyle as well.
Genocide is defined as the deliberate and systematic extermination of a national, racial, political, or cultural group.
The Holocaust was a genocide.

Answer 2

Because Hitler was killing people based on their genetics.