One simple reason: Double-standards. We don’t like to talk about it, but it’s definitely implied.
Quoted for the truth. It really depends on each person’s favorite characters. For example, a fan of Rumple is going to be a lot more apologetic and defensive about his actions, while not being so sympathetic when other certain characters do harmful deeds. Especially if those actions are targeted against Rumple. Really, you can’t make it a fact that Rumple is less evil than Regina or Hook, or vice versa. Each one has committed villainous actions on the show, and it all comes down to which one you have a personal attachment too. So arguing which one is more justified in their actions will just go in circles.