I shot the first doctor in the face too, figuring he might do something crazy. At that point I didn't really want to kill the other two, but for some reason I wasn't seeing any button prompt so I assumed that I HAD to kill them. When the girl called me a monster I sort of internally shrugged and said, "I know."
It wasn't until afterwards that I realized I probably could have gotten out of there without killing them. But I, like Joel, was exhausted at that point (I was entering my 13th straight hour of the game, with a few small breaks), and wasn't thinking clearly.
Amazing game though, really. For not only making me hate Joel for the monster he was, but also for forcing me to realize that that monster was, in a large sense, me.