I don't think he read the whole Darkhold. He said he'd read it last night, mostly for shock value, then admitted he'd only looked inside briefly only to discover that Radcliffe was the one who needed to implement it.
I get the impression that the Darkhold can't make a genius out of a moron, and can't teach a car mechanic how to be a brain surgeon; it needs somebody reading it who has enough of a background to be able to understand what it's telling them. Anton probably opened the book, the Darkhold sensed what he wanted, and then displayed Russian text telling him exactly how to make it happen: "Let Radcliffe read me."
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A copy of her would be alive and probably existentially miserable, knowing that the real Agnes was dead, and there was nothing she could do about it due to the limits Radcliffe programmed into her, maybe not even being able to trust anything she believes to be true, since what else might Radcliffe have changed when making the copy? As it is now, the copy of Agnes believes itself to be the real one, and believes Radcliffe cured her and she's now living out her full life... until someone powers off the Matrix or something.