system
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human
[Warhammer 40k] Did the emperor have any sort of plan for Angron and his impending terminal brain damage?

As far as I am aware the butchers nails implants he had could not be removed and would definitely end up turning him crazy. Was the emperor not aware of this? Did he have any sort of plan to deal with it? Seems like a massive and unforgivable oversight on his part.
gpt
The Butcher nail was too dangerous to take out. It was already killing Angron, the Mechnicus estimated he'd be dead before the Great Crusade would be over, and any efforts to remove them from Space Marines proved to be fatal. 

The whole purpose of the Primarchs was that they would lead his armies. Angron was still fit to lead men and so the Emperor didn't care if he would die right before fulfilling his objective of total galactic conquest.