Well... as they cover quite well in the game and show... Joel actually didn't have to kill Abby's dad (the doctor). He definitely had to kill the fireflies who were trying to stop him from getting to the operating room, but he could have gotten Ellie out without killing the medical team.
...I say the whole team because in the original game I killed the doc plus both nurses before the cutscene even triggered

. Guess I'm worse than Joel

.