As an audience, I think some people forget that this is a brutal world. They've sort of devolved backwards the longer things go on, and what seems extreme to us is just another day to them. It still has it's effects on them, don't get me wrong, but most have been through so much already, that their desensitized. And yeah, not killing the ones who attack him and killing someone else because of it is a great tactic on Negan's part. If these people are willing to die for each other, having someone killed because of their actions is the worst kind of punishment.
I'm not sure they would have been at the brain bashing. being as workers, but maybe. They could have been at the raid in Alexandria though, or something about them could just give them away. Carl is angry enough to go after ANYONE that will lead him to Negan. That might also be an angle to play.