The darkness obviously affected him. He went from being a coward, but otherwise a good father, to not only a coward, but a ruthless human being
The darkness did clearly affect Rumple, no denying that but look at the degrees. You called Rumple ruthless but I strongly disagree with that, especially in terms of Rumple vs Hook. After the initial taking on of the DO’s curse and the first few months, Rumple did not murder anyone to our knowledge (you can speculate but canonically within the story, he did not). Instead he chose to make deals. He chose to give people what they wanted (or thought they wanted) in return for other favors. Yes, he manipulated the HECK out of people and situations but always played into what those people claimed to have wanted and always gave them a choice.
He did not summon up a bunch of Former <del>Jawas</del> Dark Ones or open the <del>Hellmouth</del> portal to the Underworld. He did not mark anyone for death. He did not use the combined forces of the Dark Ones for any nefarious purpose. Hook, within about 10 mins of becoming the new DO plotted to betray his girlfriend, betray his girlfriend’s family, and committed cold blooded murder. Once he remembered he was the DO, his emotional attack on Emma was far worse than anything we ever saw from Rumple or Emma–at least subjectively, in my opinion. I can never forgive Hook telling Emma that she’ll always be an orphan and Emma just standing there taking it, being his emotional punching bag. Rumple could be cold or manipulative but you never saw him talk to Belle that way when he was the DO, either in the EF or in SB. In fact, in a lot of the instances of interaction between them, Belle had the power by standing her ground and standing up to Rumple, calling him on his nonsense. But the message OUAT wants us to take away is that only one of these characters is “good” and “worth saving” and that the CS ship is healthy and laudable.