Becoming Someone Else
It could have been when Kaylee had been laying on the cargo bay floor, bleeding from a gut wound, dying from a gut wound, while Simon stood eye to eye with the captain and haggled with her life.
It could have been when Simon jumped off the cat walk and ended up holding a gun at the law man, keeping him at bay, keeping him from River and realized for the first time, he might have to kill someone.
It could be when Simon planned that heist on Ariel. The idea of stealing from the sick to give to the sicker hadn't bothered him in the least. Or later, in the holding cell, when Simon pressed down on the guard's throat while Jayne killed the other one, he thought, please pass out before I have to kill you.
It could have been when Simon shot at a man with no thought of where the bullet would go. All he wanted was for his bullet to find a target and that he'd be alive at the end of the day. Even when the Shepard had assured Simon it was unlikely he actually wounded anyone and Simon felt relief, there was also a bit of hurt pride.
It could have been when Simon delivered a baby for the first time as the primary, in a bedroom like a midwife, while a gun fight went on outside and then watched, impassionate, while the new mother shot her baby's father in cold blood and all Simon could think was good riddance.
It could have been when Simon actually killed for the first time. Yes, they had just been Reavers and they were going to do worse things than kill. Just a Reaver but once a man. If Simon could have cured them of their madness, he would have. But he couldn't, so he killed them.
It could have been any of these times but it wasn't because Simon knows exactly when he stopped being Dr. Tam and became someone else. It was the moment Zoë hollered, "Fall back!" and Simon obeyed, taking Kaylee and the gun but forgetting his medkit behind.
Disclaimer: Yeah, I own nothing, 'nuff said.