They only blow up accidently.

ISIS is a small population of Muslims too.

If you compare the damage done by isis and the ones done by others over the timeframe that OP is talking about, isis would be minuscule. It only feels big because it's happening in the present. And you also need to take into consideration the effects of technology and globalization.

Atheists are also a much larger group of people than ISIS with access to more knowledge and could cause quite a bit more damage if they wished. ISIS is a pretty low threat yea but Atheists are hardly an extreme minority - they equal the black population in numbers within the US.

You can't compare a group whose sole purpose is to kill people with a group with very diverse thoughts