Personally, I just let it happen.
I'm playing an old (+-60) Imperial mage, who - after years of profiting from his passed-away father's fortunes - decides to go out and actually see the world outside of his Chorrol mansion (alternate start mod).
After a while, while working on an assignment he had received, he accidentally missed one of his more effective spells. This caused him to kill an ally, though the battle continued and he couldn't undo what happened anyway - it was an accident.
It was an honest mistake to kill the fella, but he just shouldn't jump in front of a spell named 'kill - potent'...
More info about the quest during which this happened:
Spoiler While working for the Arcane University, you'll get a task from the count of Skingrad. You'll have to get rid of both a nest of vampires and a bunch of vampire hunters. I told the hunters about the nest, and came along to exterminate the vampires.
I killed the hunter-leader by accident, and the other hunters were killed by the vamps.
The most fun part was that when Lucien LaChance showed and spoke to me, I killed him with the very same spell.
My character was pure-of-heart, after all: he couldn't let such an opportunity slip.