I have been an atheist for nearly 18 years and have debated many a theist. One of the things I find most annoying about debating theists about matters of morality etc. is that they love to play a silly childish game I like to call the blame game. Atheists themselves have been known to also fall into these stupid arguments that in my opinion are completely irrelevant to the issues that matter.
The bible actually contains a very good version of the blame game in Genesis 3:11-13 after having eaten from the tree of knowledge and attempting to hide from the presence of the Lord, they are asked by God why they had disobeyed his commandment. Adam blamed the woman and the woman blamed the snake, but in the end God made all three of them accountable for their individual part in the fiasco.
I have heard Christians blame atheism for atrocities committed against humanity by Communists or other tyrannical forms of government. Atheists on the other hand like to pull out all the dirt that they can find on theism from their own atrocities committed against humanity such as the crusades, inquisitions, etc.
What we all forget is that these acts are not the result of a particular belief system, but rather the result of the acts and beliefs of individuals. You can't blame an entire belief system or lack thereof for the acts of certain men who claim to be proponents of those systems. This also applies when they don't claim to represent any system of belief as well. People have the bad habit of labeling others by their actions or former beliefs.
In the end we as individual are all held accountable and must suffer the consequences or rewards of our own actions. That is part of being an adult, taking responsibility for the things you do in this life in the here and now not in some imaginary afterlife. I am not moral because the bible tells me to be moral and to love my neighbor etc. I am moral because society has taught me that there are rules that we all must abide by.
I didn't learn right from wrong from the biblical god, but rather from my parents. The society I came up in has laws that one must abide by or else suffer imprisonment or in some states the death penalty. When I debate theists I try to avoid playing the blame game because it draws attention away from the real issues that need to be discussed. I can't blame the person I am debating for his religious institutions immoral and brutal actions that happened in the past and in some parts of the world continue to happen when he personally had no part in it.
I personally have a felony on my record from 2003 for resisting arrest one night and injuring a police officer in the process. But society labels me a felon and puts me in the same category as a rapist, murderer, pedophile, etc. What happened to me could've happened to anyone but I have and continue to be judged as bad because of the label felon. It is difficult for me to find a job and I am discriminated against for being a felon. Although I have never stolen anything or robbed anyone or committed any major crime in my life.
In conclusion my fellow atheist, when dealing with theist try not to make blanket statements or label and discriminate against them for the things that other rotten apples of their religious institutions have done. Not all theists are bad, and some are very open to reasonable discussion regarding matters of their faith. Sometimes in the end you just have to agree to disagree and let them go on with their delusion. And remember always to be responsible for your own actions whether they be for better or for worse.