I'll go ahead and say it was Adam. He wasn't deceived about the significance of eating the fruit, yet he went ahead and ate the fruit. He just didn't care that much either way. I've always kind of assumed that he thought he would be Eve's ruler, "knowing good and evil" [beneficial and non-beneficial, for them], at least as far as she knew.
But, unbeknownst to him [because, perhaps, he was just too lazy to think it through] he was appointing Satan as ruler. Apparently this was the situation here: 1 John 5:19.