"You are innocent, as the stars say. And to slay something innocent to save oneself, that is a terrible deed. One would live only a cursed life, a half-life, from that day. For any centaur would surely be cast out, if he slew a foal."

The spear made a lightning motion, too fast for Harry's eyes to follow, and smashed his wand out of his hand.

Another powerful blow smashed into Harry's solar plexus, and he went gasping and retching to the forest floor.

Harry's hand reached up toward his robes, for his Time-Turner, and the spear-butt knocked his hand away, almost hard enough to break fingers, he reached with his other hand and that was knocked away too -

"I am sorry, Harry Potter," the centaur said, and then looked up with widened eyes. The spear spun about and came up, intercepting a red spellbolt.

The centaur talks about Harry's apocalyptic destiny. Then he talks about the implications of a centaur killing a child. Then he attacks Harry. Then, once Harry is fully disarmed and vulnerable, he apologises. Then he's surprised by Quirrell's assault.

You appear to be offering two alternative interpretations.

a) The centaur talks about Harry's apocalyptic destiny. Then he talks about the implications of a centaur killing a child. Then he attacks Harry. Then, once Harry is fully disarmed and vulnerable, he apologises. But he's just disarming Harry for its own sake, with no intention of killing him, because... he felt like it?

b) In the middle of the conversation, Quirrell Imperiuses the centaur to attack Harry, and to apologise in the middle of doing so, before coming to the "rescue" himself. The fact that the centaur was talking about centaurs killing innocent children right before this is a complete coincidence.