It's kind of an intrinsic thing my people have. Time Lords can see all of time, and we know what are fixed points and what aren't. And yes, it is because of the things he did, and also what the world would ultimately learn because of the destruction he wrought. But it is not limited to people. Pompeii being destroyed by Vesuvius when it erupted in 79 A.D. is also a fixed point in time. Some things must always happen, no matter how terrible. It is hard to take, but sometimes the very bad things can lead to very good things coming about in response to it.