There is a paradox going on here. We need to start from some assumptions to see it:
- Volkov's bullet and gun is not reversed, from Volkov's own perspective the bullet is shot out of his gun into Neil's head.
Let us look at the event from the forward perspective:
- TP and Ives get to the hypocenter, door is locked.
- Volkov reaches the hypocenter from another door to the left.
- Neils body is already on the floor
- Volkov shoots Ives
- Volkov goes to shoot TP and a forward bullet goes through a reversed Neil, Neil should have at this point a reversed wound
This is where the logic breaks, Neil from the forward perspective is now going backwards with a hole in his head, if we assume the mentioned assumptions. This would make it impossible for Neil to come to the rescue in the first place.
How do they make it work in the movie?
By braking the rules, what we see:
- Volkov reaches the hypocenter from another door to the left.
- Neils body is already on the floor, with blood on the helmet
- Volkov shoots and the blood disappears,
- Neil is seen moving backwards, uncanny to the whole thing
Now, the paradox:
From Neils perspective a bullet goes through him into Volkovs gun, still opening a hole in his head, that is why he dies. With Reversed bullets there is always a wound, moving forwards or backwards in time the wound is still there.
We see this with Kat, Kat is shot by an inverted bullet that goes through her, but she is still injured at the end. If both the bullet and her were moving in the same direction, then we would see, looking at the scene in the opposite direction that a bullet is going through her healing a wound in the process. But that is not the case.
By the same logic, Neil would be both death moving forwards or backwards when the bullet goes through him, erasing his past and future, hence the paradox, does Neil exist or not?