I think you answered your own question there. I think the hate towards Milah stems from the fact that Rumple is loved by so many fans. It really started to snowball after Desperate Souls and we learned why he became the Dark One and also later, why he needed the curse to come to our world. He became more sympathetic. So anyone who hurts him, fans automatically hate. Like Milah and Hook. But IMO, Milah’s a lot easier to hate because we know so little about her. We know almost all of Hook’s backstory. There’s very few gaps.
(emphasis added since it’s really the point I want to discuss)
I see this idea a lot–the idea that you hate a character because of what they’ve done to another character, especially if it’s your favorite. I do think that plays apart but I’m not 100% sure I agree. I can dislike/hate the actions of a character even if they are done to a character I don’t enjoy or am indifferent too. Actions are actions, be they done to my favorite or my least favorite. Let me give you a totally bizarre and random example. (and just as a clarification, I do not hate Hook, not even in the slightest. I have a different perspective than Hookers and/or CSers. But this is the easiest example I can think of at 8:30 am)
I like Belle. I find her interesting and compelling and she’s one half of my OTP. I do not like the physical assault visited upon her by Hook. But let’s say it wasn’t Belle–it was..I don’t know..the Blue Fairy. I still would have disagreed with/been troubled by the action itself and that of the agent in question, even if I think Blue is secretly the evil mastermind behind all the things. In other words, it’s the action and the actor rather than the person to whom it is done.
Or, conversely, I wasn’t overly fond of Rumple beating the stuffing out of Hook on the Jolly Roger. But that’s because I am opposed to physical assault and violence in all forms–even if it’s from my beloved Imp. In that moment I disagreed with the actor (Rumple) and the action (the beating) while feeling more sympathetic toward Hook.
I still would have disliked Milah even if it wasn’t Rumple and Baelfire she abandoned because of the simple act that she abandoned her child. Is my sense of sympathy doubled for Rumple because of Milah–yes. But if she had just been a no name peasant who up and left her 7yr old, I still would have disliked her.