If someone decided to believe something completely wacko (like scientology) and then called it a religion; does that reflect meaningfully on the other religions? If someone called themselves a "Christian" or "Buddhist", but then did something obviously un-Christian, un-Buddhist, and un-nice, does it mean that Christianity or Buddhism are necessarily evil or un-nice? The same can be said for atheism. Hitler was not nice; if he had applied Christianity or Scientology, would he have turned out better? No way to tell.
I would say that you have to evaluate each person individually, and how they apply their religion, and then see whether it made a positive or negative difference in that one case. Whether supposedly "religious" people either do or do not behave better or worse than their neighbors doesn't have much significance beyond their specific case. Even then, it is hard to know how they would have turned out otherwise.
No one ever hears about the religious people who didn't become serial killers because of their religion, precisely because they didn't, and there is no way to know that they would have.
Every generalization is wrong.
The label "religion" poisons (or at least confuses) everything.