Yes, she was not mother of the year and left Bae, and she could have openly leave instead of pretty much sneak away. She did Rumple wrong. Okay. What I don’t get: how people can develop so much hate for a (supporting and minor) character based on so little information.
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.