Rescuer

The person who shot Killer and ended the rampage.

The irony here is that he isn't a gun nut: he passionately dislikes them. In the frequent arguments on the topic within the company, he's usually been the strongest advocate of gun control.

But when he realized what was going on, in a desperate moment, he remembered that CEO always bragged about keeping a gun in his desk. So he dashed in there, grabbed the gun from the desk, and fired fairly wildly at Seamus. He hadn't even wanted to kill him, but emptied the clip into him, in a panic.