How did one man cause the Holocaust?

How did one man cause the holocaust?
Did any people protest against the Holocaust?
Why didn't other countries help the Jewish people?
Was only Jewish people killed during the Holocaust?

This is what i already know about the Holocaust.. that Adolf Hitlar was the leader and turned almost everyone against the Jewish people. Also that they would take the jewish people to concentration camps and most of them died in these camps or got out alive but with a experience they will never forget. But I want to know how this Adolf hitlar got all these people to be against one race of people. Also what would they do to the people that tried to stand up to adolf?