"Tendency towards overkill," he says crisply, "literally. Apparently it didn't occur to him that 'defeat' might just mean 'defeat,' and not 'annihilate.' I killed the Headmaster after the wand had been taken from him, so as far as the wand was concerned, I was irrelevant."
"Says you," he returns blandly.
He sits, too, and starts a flowchart in the sand with the narwhal tusks. "Because time isn't as simple as we might prefer. That's why time-turners are so strictly controlled. We may think it's past to future," he draws a line, "but choice matters. Maybe other things, as well." He draws branches out. "Suppose I hadn't set the Hat on fire? Things would be different. Or suppose Helga Hufflepuff had been abused as a child and grown up twisted? Everything would be changed. We know this history," he taps the first line, "but others, in this place, may know this one," he taps another, just parallel to it, "or even this one." This time the line he taps is far off the first one, and so many branches away from it as to be very nearly perpendicular. "Everything they know will be different. Things were different there--events, and people. It's like that."
BOO to you for making me explain the Trousers!! XDDD