Why were people killed during the Holocaust?

people were killed during the Holocaust because Hitler didn't want anyone interfering with his plans and because he believes that some people were an inferior race to Germans such as Jewish, Catholic, homosexuals, Jehovah's Witnesses to concentration camps where they would do back breaking labour, treated like slaves and when they were of no use anymore they would be put in gas chambers, which were located in extermination camps, and they would be killed.