Edmund didn’t like Goneril. That was the only reason this would work at all. Of course, he didn’t like Regan either, but his dislike for Lear’s eldest daughter set any other in the shade.
(He did like Edgar, for certain definitions of ‘like’ that included a fierce and unrelenting desire to fuck him against any available mattress, wall or other sufficiently sturdy surface. Fuck his chaste, tight, gloriously muscled ass until that prosy mouth groaned his name and begged for more, harder, faster, ohletmecomepleaseEdmundletmecome. There were even times when he entertained the thought of being thoroughly fucked himself, an idea he usually dismissed as more akin to nightmare; except that he woke from those dreams hard and ready for a fight. Usually with Edgar.)
He knew he was beautiful, blond and slim and elegant as Goneril was not, never had been, never would be. He knew how to use that, seduce her with his charms, his wit, his flattering attention. Let her think the plot was hers, the clever machination by which to overthrow a father foolishly and over-fond. He would never have that problem.
The seduction came off perfectly, Goneril then Regan (seduced well by her sister, needing no help from him, thank whatever stars you will) falling to his hand. The map divided, the clever daughter exiled to France - that was a piece of Fortune he had only hoped for: Cordelia was far too perceptive and clear thinking for his comfort. Of course, then everything began to fall apart, and neither brains nor beauty could repair the breach.
The climax came when Edgar pierced him, finally, completely, close and hard and feeling everything. It was a consummation devoutly to be wished; Edmund could die now - little death or big - with speeches or without, impaled on that unrelenting blade.