He chose to kill the Dark One. He chose to take the powers. He didn't choose it for the power though. He chose it for his son. And that's what the problem is. The curse is angry, oh so angry. It just isn't fair. This is the least control it's ever had on its host, all because its host didn't choose power, he chose his son. Now he won't give in to it completely. Oh how it wishes the host would let them kill the boy, but he won't. Choices have consequences, and this man made the wisest choice any Dark One ever has in choosing his son and not the power. As a consequence, he has more control.
But all magic comes with a price, and it realizes this when his host experiences True Love's Kiss. He should now have to choose between love and power, but there is a problem. The host loves his son and he needs the power to get to his son. He can't choose between love and power because his love for his son makes him need power. So he refuses to choose at all, not giving in to true love, but not letting it take complete control either. So now the man has a problem; he can't make choices.
Time and time again comes the supposedly black-and-white power vs. love struggle. The man can never pick a side. He loves Belle but tries to kill Regina despite her wishes. He manipulates and toys with people using his power, but he still loves Belle, no matter how many times she walks away. He won't kill the boy who is to be his undoing, even if he tries. He won't because he can't give up love, not even for power. And even when he dies, a death that by all rights should have caused the curse to break because of the purity of the sacrifice, the man held on. He held on to the only thing he'd had for centuries and it took him to the vault with it. With it and the one other thing the curse had to begrudgingly admit had always been there for its current host: love.
The witch and the death of one the host loved so much tips the scales in favor of power. Love is still there though, in the form of the man's wife. No other Dark One had ever married before, and it's angry. She should be killed. She is dangerous. It is proved right when they stand by the town line. It finds it ironic when she says the man chose power. Doesn't she get it? The man is incapable of choosing. He has been ever since it met him. The host says they could have it all. He can't choose. And then Belle is casting the host over the town line and it has to brood, stuck in the dagger. It isn't fair. If the host had just chosen power, none of this would have happened. All these foolish people would be dead. That one choice oh so long ago, the one where he chose power not for power but for love is the problem. Because now love and power are impossible to choose between. The host really can't choose. Love and power are entwined for him in a way they never have been for anyone. He won't choose power over love, and it refuses to let him choose love over power. So he doesn't choose; he tries for both. And that's the problem.