Why is it so many atheists define God based upon the those who did evil in God's name?

I absolutely agree. I may claim to kill in MLK's name, but it would be all me - nothing to do with what he actually taught. Likewise, people throughout history have claimed God because it made them look good in the eyes of the people or because it lent an air of credibility or power to what they were doing. God did not order or condone what they did - it was all them.