An interesting thought, and one that I had not considered in this light. And yet, right afterwards:
The old wizard looked back up at Harry, and said, in a hoarse voice, “He is not like Grindelwald, Harry. There is nothing human left in him. Him you must destroy. You must not hesitate, when the time comes. To him alone, of all the creatures in this world, you must show no mercy; and when you are done you must forget it, forget that you ever did such a thing, and go back to living. Save your fury for that, and that alone.”
This seems to make it clear to me that it is not that Dumbledore thought that he would never kill, and then Voldemort’s war changed that, and he killed. It’s that he thought that he should never kill, and then Voldemort himself became the exception, changed the rules so that he should kill Voldemort.
An interesting thought, and one that I had not considered in this light. And yet, right afterwards:
This seems to make it clear to me that it is not that Dumbledore thought that he would never kill, and then Voldemort’s war changed that, and he killed. It’s that he thought that he should never kill, and then Voldemort himself became the exception, changed the rules so that he should kill Voldemort.
At least, it is an alternate possibility.