Season Four:
Jack enters an apartment to secure a laptop, unaware that one of Marwan's female operatives is there, impersonating an FBI agent, having killed a couple of agents before Jack got there.

Jack eventually figures it out and the chick takes a shot at him, missing. You can almost see a "doesn't this bitch know who the fuck I am?" look on Jack's face before he fires two shots and kills her through the drywall.

Also from Season 4, even though it isn't anything Jack did, I still like the moment. After securing Dina Araz, (Shohreh Aghdashloo), Jack is trying to make a deal with her to turn Behrooz in. Dina says the only way she will tell them anything is if they find her son because she doesn't care about any American lives and firmly believes in her cause. Without missing a beat, Jack deadpans to Driscoll "Did you get that?" and the episode ends.