SPOILERS (HUGE SPOILERS)
I've been non stop thinking about how he actually killed people and managed to get away with it. There's 3 ways I have considered.
WAY ONE:
World A is dominant:
Fisher assasinates a Person for some reason or the other and then waits until 9:22
World B is "organically" changed to align with World A.
I think the way it aligns is strange, so in World B they die from something else (because Fisher doesn't exist in that world, so how could he have killed them?) i.e. a car-crash.
Then Fisher moves to World B and makes it dominant. He doesn't do anything and just waits for it to resync. This means that in both worlds Fisher had nothing to do with the murder.
This is actually terrifying too, because one version of them gets written out of reality in the same way Spencer did. Forever. They'd have no idea how to fix it. It also explains why the "Other Spencer"'s death was suspicious, because to cover it up, Fisher would've had to let his grandson die in both realities.
WAY TWO
It's implied that if you die in the tunnel then you get fully written out of history. In that case, all Fisher has to do is lure them to the tunnel and kill them.