You cannot kill Daemons. They exist in a void as thoughts, or abstract notions, through which the only way of entry to our world is the Polar gates, created by the Old Ones and perverted and destroyed by these capricious entities.
The greatest of these abstract notions are Fear, Anger, Want, and Power, the 4 chaos gods, and the lesser ones are bound to them. It is these Chaos Gods who can tie these lesser daemons to the physical plain in a form that's pleasing to them (Personally, I think Khorne became gay), but they only have a need for these in the physical plain.
They can only ever be banished to that realm, never destroyed.
The Old Ones sealed the fate of the entire Warhammer World when they created these transportational gates, and encroached upon the territory of these daemons, angering them.
As to power, I don't think so. He was at the height of his powers, already several thousand years old, with the teaching of the Old Ones to guide him, without relying on the plaques. Even then, he couldn't muster the power to fully stave off the assault of the Daemons, and combined with his power and the Elven Vortex of Caledor Dragontamer are the Daemons, hence themselves, and all other races (including the Dwarves, Halflings and Ogres) became weaker for it.
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