Should he have intervened?

God allowed the Holocaust for the same reason that he has permitted all human suffering: to settle moral issues that were raised long ago. The Bible clearly indicates that at present the Devil, not God, rules the world. Luke 4:1, 2, 6; John 12:31. 

God told the first humans, Adam and Eve, what he expected of them, but he did not force them to obey. They chose to decide for themselves what was good and bad, and their bad choice—and similar choices by people throughout history—has brought horrible consequences to mankind. Genesis 2:17; 3:6; Romans 5:12. 

It is as the book Statement of Principles of Conservative Judaism says: “Much of the world’s suffering directly results from our misuse of the free will granted to us.” Rather than revoke our free will, God has given humans time to try to manage their affairs independent of him.

God can and will undo all the damage of the Holocaust. God promises to bring back to life millions who have died, including Holocaust victims. He will also eliminate the pain that Holocaust survivors feel because of horrific memories. Isaiah 65:17; Acts 24:15 God’s love for mankind guarantees that he will fulfill these promises.—John 3:16.

Many Holocaust victims and survivors were able to maintain their faith and find meaning in life by understanding why God has allowed evil and how he purposes to undo its effects.

Many who ask these questions have suffered extreme personal loss and seek not just answers but solace. Others see the Holocaust as the height of human evil, and they struggle to believe in God.

Back in the days of Noah, God “felt hurt at his heart” because of the violence spreading in the earth. Genesis 6:5, 6 No doubt, God also felt tremendous pain over the Holocaust.—Malachi 3:6.

 God did allow Jerusalem to be destroyed by the Romans in the first century. Matthew 23:37–24:2. Since then, however, God does not single out any ethnic group for special favor or for punishment. In God’s view, “there is no difference between Jews and Gentiles.”—Romans 10:12

Although God never causes suffering, he sometimes allows it temporarily.—James 1:13; 5:11

What he overlooked is that mankind's "disasters" are brought on by mankind.

What kind of intervention did you expect? Christ would descend from the sky, admonish Hitler and raise the dead? Germany was reduced to rubble and divided. A once notable economic power into a powerless state that even now they haven't recovered from. The mere mention of their name brings back images of fascism and destruction, even in the movies the bad guys have a German accent. It's a historical fact that on the night before a great storm was coming in and the allied forces would have been stopped from delivering a decisive blow to the German military, General Patton knelt in prayer and asked God for clear weather to enable the them to strike against Germany. The next day, the weather was clear and sunny. 

Why does God allow suffering? Watch this video: https://www.jw.org/finder?wtlocale=E&docid=502018850&srcid=share

God does not now miraculously intervene in people’s lives as he did on occasion in Bible times.

If he existed he should have intervened. It would have been the right thing to do. Allowing people to die when you have the power to save them is evil and certainly irresponsible. 

Since God and Jesus are fake, they can't do anything.