Even if Hitler was an atheist, so what? Hitler was also a vegetarian. Does that suggest that vegetarians have a special tendency to be murderous, bigoted racists?
The point is that there is a logical pathway leading from religion to the committing of atrocities. It’s perfectly logical: if you believe that your religion is the right one, you believe that your god is the only god and you believe that your god has ordered you through a priest, or through a holy book, to kill somebody, to blow somebody up, to fly a plane into a skyscraper, then you are doing a righteous act. You’re a good person. You’re following your religious morality.
There is no such logical pathway leading from atheism, or secularism, to any such atrocious act. It just doesn’t follow.