This is a ridiculous statement. I have the same concern for my fellow man as an atheist as I did when I believed in a higher power. You have no way of backing up this position.
And that may be true for you, but you have no way of backing your statement up either. We'll just have to take your word for it. But the fact still remains, nevertheless. History proves that atheists in power are more likely to rule ruthlessly.
The problem with your reliance upon ancient history to prove religion is harmful, is that you do not have an atheistic civilization to compare it to. Virtually everyone was a theist. Sure, some people killed in the name of religion, but many other showed forgiveness and mercy in the name of religion.
Those who killed in the name of religion, could have very well done the same or worse, if they had no religion. Religion is often used for political purposes. For example, the Roman Emperor Constantine used Christianity as a political tool. The Church itself had to bend to his will. Now you talked about the inquisition, but the fact is relatively few people died during the inquisitions and these forms of justice proved to be the most enlightened of all other systems being practiced throughout Europe at that time. Would an atheist ruler have come up with better system? Maybe. Maybe not. Until you provide an apples/apples comparison, you're just speculating.
Atheists have since sporadically risen in power and they have not shown us that their ways are less dangerous. Power and money corrupts. It just happens to be more prevalent in atheist rulers, and I think this has something to do with their lack of belief in a higher power. Without that belief, there is no consequence either in this life or the next.
And since we're on the JW subject, you should also remember that JWs refuse to murder, period. Under any circumstances, they refuse to serve in the armed forces. How does that make them dangerous to us?
“All knowledge of reality starts from experience and ends in it...Propositions arrived at by purely logical means are completely empty as regards reality." - Albert Einstein