Question

how did Hitler and the Nazis justify the Holocaust in other words why did they kill Jews and other unwanted group

Answer 1

There were so many (???)

Answer 2

Hitler believed that there were too many Jews and unwanted groups in Germany and that they were loosing control over it he wanted it to be just Germans as he would say a " pure race". So what he decided to do was to kill all unwanted groups that he thought shouldn't be there or had no purpose. I hope this helped :)