Even if Knull could pull off a White Light attack, Nekron isn't weak vs the White Light/Entity at all. This is a forum myth. He was about to kill the Life Entity AND it's host. If you reread Blackest Night, this was mentioned on panel : (please log in to view the image)
They had to sever his link to the physical universe to BFR him back to his realm. (please log in to view the image)
I wasn't serious, but it actually raises a real question.
If it was Black Hand who was Nekron's connection to that universe, how do we treat him in vs. threads? BH is obviously not here but we can also not really act like we do with, lets say, the speedforce because the speedforce might enable Flash's ability so that the character works for a thread but by doing this with Nekron and act like there is some connecton somehow we keep the one way to defeat him quite obscure and don't really provide something for his opponent to interact with.
So we end up with either:
There is no connection and therefore he shouldn't be here in the first place.
There is some connection that can't really be tempered with because it's not defined how it works.
__________________
“I once had an entire race killed just to listen to the rattling of their dried bones as I waded through them.” —Volrath
This thread assumes they can even face off. That means an anchor is present. Knull isn't doing anything to Nekron's anchor sans White Light. Nekron kills him. Thread.
I assumed they were able to face off in a neutral universe where their powers worked (aka Flash and speed force). Otherwise what's the point of the thread?