Holocaust?

I want to know why such things could happen for so long with no one saying anything about it. I also want to know what could drive a man to be so evil so out of his mind that he would kill his own people. He killed his own people for the sheer fact of they believed in something he didnt. Just because they have different beliefs than yo7u doesnt mean that you can kill them for it. Everybody is different and that doesnt give the right to make fun of or kill the person. You dont have to like what someone else does but you have to respect what they believe in.