Indeed we are not arguing semantics.
Sure, from the murderer’s perspective, he had bechira, and could choose to kill or not.
But from the rest of our perspective, Leiby hy”d would have been murdered anyway, regardless of what the murderer chose. It could have been in an accident, it could have been by a different murderer.
But to say that if the murderer had not chosen to kill, the Leiby would be alive, is simply not true.