Not all that many; although being an atheist used to be a capital offence in England, not that many people were actually hung - largely because, obviously, people wouldn't say they were atheists.
In general, atheists don't kill people because of their beliefs - it's hard to think of atheists flying airliners into buildings or starting a war because of beliefs; why would they?
Answer:
It depends on your definition of a "cause". No one has ever run around (to my knowledge) in an Atheistic Crusades saying "Be an atheist or die". However societies with Marxist (which is atheist in practice) or general atheistic philosophies have killed millions. Stalin's USSR, Mao Zedong of China, and Pol Pot in Cambodia are three examples.
To answer the question it is necessary to determine first which states are "atheist". On examination of the world's governments it appears that there are no atheist governments (denying the existence of god(s)). Many of the world's governments are secular (many European governments, Canada and America) with a direct statement that churches or religious considerations are not part of their governing process, some ban religions without making any statement on the existence of a god (China, Russia) and some are theocracies run by religious rules and religious people (the Vatican, the Arab Emirates and Israel). North Korea is somewhat out of this list as the state has developed its own "religion" (Juche).
All of these types of countries have directed the killing of people in various wars and pogroms. None are atheist states. So atheist states have killed no one.
Exponentially less than religious people
The people killed or wounded in battle are called casualties.
People have been killed by shocks of just 50 volts
Chinese people have killed and been killed. China (earthenware) has been used to kill someone. China (the country) has not killed anyone.
There has been no strong indication that Richard Hammond is an atheist. But, neither has there been overwhelming indication that he is of religious beliefs.
In the last 10 years 100,000,000 people have been killed by electricity in the world
Over 10 years 6 people have been killed.
256 people have been killed.
There has been no strong indication that Richard Hammond is an atheist. But, neither has there been overwhelming indication that he is of religious beliefs.
Casualties?
None.
There have been very few documented cases of people being killed by clowns. While there have been isolated incidents of violence involving individuals dressed as clowns, the overall risk of being killed by a clown is extremely low.
There have been no reports of people being killed by kangaroos.