Somehow, one of my players managed to kill the player inside the nether. Now the view through the portal is shaking (As tends to happen during death) and entering the Nether crashes their game with a NullPointerException.

I'm not surprised everything broken when the camera entity died, that's never supposed to happen so none of the other code will be equipped to deal with it.
I'd be much more interested in how they managed to kill it though, it's in spectator mode and should be invulnerable.

It was a strange circumstance, but this is what I remember:
We went through to the Nether, travelled 100 blocks or so, and built another portal (after messing about with obsidian generation by me breaking the portal in the Nether and him relighting it in the Overworld).
Through that portal, he was very quickly killed by mobs on the other side, and reported his game freezing a few seconds after leaving the portal (Enough to get a few hits in on the mobs) before they surrounded and killed him. Then, he said that he found himself back in the Nether at his death Overworld coords for a few moments (He found a Nether Fortress at those coords), but his connection to the server kept timing out whenever he tried to respawn.
I restarted the server and he was able to connect again and respawn at the appropriate place in the Overworld, but his Nether Portal view was glitchy like that, and trying to travel through made his game outright crash with the error 'Error: java.lang.NullPointerException: Unexpected error'.

If it wasn't due to this sequence of events, perhaps one of the Bewitchment mobs was able to kill it somehow for some reason.

Which BP version were you using when this happened?
I fixed a few death/respawn related issues in 0.1.3. Just want to make sure it's not one of those.

I updated to 0.1.4 just before I created the world.
On the topic of damage, the view through portals will also be weirdly static while the main camera tilts when taking damage.