I have.
Of course, "kill" is too strong a word for what most doctors have done at some point, but still, he died because of my direct actions (a nurse and respiratory tech were involved, too).
He was a retired farmer, around 80. Strong and healthy for his age. I was in my early 30's, just out of residency and starting life as a newly-minted attending physician.
He'd fallen from a ladder and severed his upper cervical spinal cord. He was awake and alert, but completely paralyzed from the neck down. He couldn't breathe on his own, so was facing the rest of his life on a ventilator, requiring complete care for everything. There was no hope for recovery.
In the first days of the injury the trauma and neurosurgery people worked their magic, stabilizing what was left of his neck, converting him quickly from a ventilator tube to a tracheostomy for comfort, getting a feeding tube in.
As the days went by and we began decreasing his medications it became clear that he would live, was mentally intact, and could communicate with us.
In spite of what his family had told us, he wanted to be let go. He'd led a robust life and didn't want to spend the rest of it in this condition. He was ready to have things turned off so he could pass.
This sort of thing is (comparatively) easy in someone with advanced dementia, or severe brain trauma, or end-stage cancer. But in a guy who was fully awake and who'd been healthy and vigorous a few days earlier it was a whole different matter.
The family was unhappy, but acquiesced to him. To cover myself I had a psychiatrist interview him and ordered a consult from the hospital ethics committee. All agreed that he was pleasant, had a good sense of humor, and was fully capable of making this decision.
The family didn't want to be in the room when it happened, so we gave them all the time they needed to say goodbye.
When the time came we chatted briefly, and he thanked me and the team who'd taken care of him. He even asked that we shake his hand.
I had the nurse give him megadoses of morphine and Ativan, to put him to sleep and take away any pain that might still be there. Once he was out the respiratory tech disconnected him from the ventilator. Between the medications and his non-functioning diaphragm he went pretty quickly. I wrote a death note and moved on to another case. There are always more consults.
What would you call it? An execution? Physician-assisted suicide? Compassion? I'm sure some out there would love to have me tried for murder, but I don't care.
To me, my responsibility is to the patient. This man had lived a good life, wasn't suicidal in the sense of someone who's depressed, and made a rational decision about his own existence. I did everything I could to make sure there wasn't a good reason NOT to end his life, and did what I could to respect his wishes and relieve his suffering.
That was the first, though not last, time I've had to face this situation. We may become more experienced, but it never gets easier.
It's been 20 years, and I'd still say the same thing: In the end it wasn't my decision, it was his. I just did what was right for the patient.