I am actually a huge Hook fan, I just get stressed everytime I see that kind of plot. If Rumple was full of shit and the hand was indeed cursed then yes, I understand why his inner darkness manifested at that particular time. If he was telling the truth I just didn't get why would his inner darkness manifest now, when everything was going so great for him. The woman he loves has finally opened up to him ans is ready to have a relationship, he's been accepted by everyone. Why now?

All right, darling. I apologize for misreading your first message. 

The simple truth is that Killian Jones has low self-worth. And I know that sounds ridiculous since the man is always boasting about how handsome he is and his sex appeal and how he’s good in a fight, but really, those are the only things he boasts about. That’s where his confidence lies. He knows he’s hot, he knows he can satisfy a woman in bed and he knows he can kick anyone’s ass.

But there is a darkness in him and a self-loathing that makes him question his place in Emma’s world. He’s done terrible things. He believed at the time and maybe even now, that he did them for the right reasons, but that doesn’t make them any less terrible. That doesn’t make him any less of a pirate. And he loves Emma so much and he wants to be the best version of himself for her and he is, but because he has so many misgivings and so much guilt about the stuff he’s done in the past, it’s very easy to trigger his fears.

When we first heard that Killian would get his hand back, there was an ugly debate about it - whether it was sending the wrong message that he needs a hand to be whole. But it turned out that wasn’t the reason he wanted it at all. He wanted to be able to hold Emma with both hands because he believes that she deserves that - a man without a hook, a tool he’s used to bring fear and pain to his enemies for 300 years. He wanted to go as the best version of himself, and it wasn’t about needing a hand to be whole, it was about not having the reminder of the man he was hanging from his wrist. Same thing with his clothes. He changed them to distance himself and their date from his past because he’s moving forward with Emma and finally finding happiness. 

And that is exactly why now, to answer your question. Because Killian was honest with Rumple when Rumple asked why he wanted his hand back and then he took advantage of that. Whether or not the hand was actually cursed doesn’t matter at all because Rumple put the seeds of doubt in Killian’s mind. He reminded him of how he used to behave and then he took advantage of his weak moments to force him into getting in even deeper - just like he did with Anna in the past. And Killian might not have Anna’s innocence, but he’s a man in love, wanting the absolute best for his love, and Rumple took that and manipulated it and used it against him, just like Zelena did when she cursed his lips last season. 

It is frustrating to watch because we know that Emma and the Charming family would support him in a heartbeat if he’d gone to them and told the truth right away. Even if he does it now, they’d still support him and believe him because he’s proven himself and they care about him. But Killian doesn’t have the faith in himself to believe that. He knows they’re good people, but he still doesn’t believe that he deserves to have a place with them. And Rumple’s counting on that. He’s counting on his fears and his doubts so that he can continue to manipulate him into doing his bidding.