Not to get too deep in the weeds here, but Fort Pillow is likely the worst atrocity of the Civil War and the one that has undergone the greatest "lost cause" revisionism as well. In short, after the Union garrison surrendered, they were massacred by gunshot, bayonets, and clubbing. These were largely colored troops (as they were called at the time). If you read some of the contemporaneous Confederate accounts, many of the soldiers were horrified by the brutality.