After Xavier revealed that he had killed for the first time and told his 5 unit leaders, including Zack and Karim, that they may have to do the same in the future, there was a moment of silence. The atmosphere was difficult to read, but it eventually became awkward with them all sitting around Xavier, who was injured and had just been interrogated by them.
They hadn't taken the time to consider how he had felt, which they regretted. And after hearing that he hadn't managed to sleep and had done it with the Outlaw Gang's best interests in mind, they felt terrible. In his situation, they likely would've wanted to do the same but would have also struggled to do so. Remaining calm despite having his life on the line and fighting trained assassins in a pitch-black environment was not something that many could claim to be capable of.