In Ten Forward, O'Brien joins Daro for a drink. He apologizes for his behavior on the turbolift, which Daro accepts, and indicates that it's taking a toll on both sides. O'Brien then brings up the Setlik III massacre. He was sent there to reinforce the garrison there, and that mostly everyone was dead. Daro tells O'Brien that they were told it was being used as a launch site for a massive strike against them. O'Brien was with a group of women and children when two Cardassian soldiers burst in. He stunned one, and was jumped by the other. During the struggle, a woman tossed him a phaser, and he fired. The phaser was set to maximum, disintegrating the soldier. O'Brien had never killed anything before then, not even a mosquito. Finally, he gets up and says to Daro "It's not you I hate, Cardassian; I hate what I became because of you". The Wounded (episode) Comes to mind this morning.