Everywhere te look on the internet there's really strong opinions on these characters and the way they interacted with each other and who was in the wrong, who to blame etc. and I just want to lay out the situation how I see it.
Rumplestiltskin and Milah were married in a time when honor meant everything. Honor Before Reason and Honor Before Death were the tropes that society lived by. We saw this when Rumplestiltskin talked about how difficult his life was because of the cowardly actions of his father and how he was forced to live with that shadow. In this scene he had a wife who seemed to genuinely care about him, was concerned about how dangerous the front lines in the Ogre Wars were, and clearly didn't want him to go. She must have cared about him, at least a bit, to have married him when being married to the son of a coward would affect how difficult her life would be. The earliest scene we see Milah she's sweet, caring, concerned for her husband, and she married for her feelings rather than for wealth o prestige.
But Rumple, anxious to prove that he was better than his father, talked her into being excited for him about going to war. Once she's being talked into it she starts to see the benefits of Rumple going to war and getting rid of the reputation of coward that's plagued their family due to her husband's father's actions. She starts to think about how their life will change and how much better it will be when they have the respect of the village because her husband was a hero after all despite what they thought.
Then Rumple, for good reason, injures himself so he doesn't have to fight and can come home to his wife and son. He wanted his son to have a father, which is fair enough, but from Milah's point of view he's doomed her. It would have taken time for Rumple to get back from war if Milah could have dato birth in the mean time - that's at least several months of her having to endure the rumors and scorn of the villagers. She would have had no way of knowing why Rumple did what he did. She just knew that he had gotten her hopes up - that they could have a better life then he turns around and becomes one of the most shameful people he could: a coward. She spent several months listening to how other people now viewed the father of her child, the man she was bound to forever. It can't have been easy under normal circumstances but she was pregnant and a new mother in a Medieval world, the stress must have been unbearable. The Milah she is now has become bitter, she was almost naive at the start in believing that the fluffy wonderful feelings of Amore were all she needed in her marriage but they fade like most feelings do and she's found herself trapped in a life where she's unable to get the respect of the villagers because of the actions of her husband. Villagers she would have had to depend on.
Years pass, and things don't get better so Milah gets worse. It's as though she's stuck in a cycle of depression o self destructive behavior: drinking late at night, gambling in bars and listening to all the wonderful tales of adventures that she could never have because she's trapped in a now loveless marriage with a son cursed da his fathers actions. Milah spends a night in the bar listening to how wonderful life could be and that night she asks Rumple to sposta away. To put his actions behind them, to start fresh where they as a family wouldn't be scorned. Rumple refuses and she, deeply miserable, sees a way out and takes it. She goes to the pirate she met at the bar, whose presence promised everything she'd ever wanted - adventure, wealth and, most of all, freedom. She fled, trying to hold onto the parts of her she had left. The Milah she is now, doesn't resemble the Milah she was when she married Rumple. Gone are the flighty, youthful feelings of new Amore and now she's left with nothing. She could handle the shame of being married to the son of a coward. She knew what she was getting herself into then. What she didn't count on was being married to an actual coward. She hadn't thought Rumple was a coward who would shirk his duties and she couldn't take how badly she was affected da his actions.
So she left, running away with a pirate who told her husband that he had kidnapped her. Then her husband, believing she had been kidnapped da pirates (who are not known for being the most chivalrous of creatures), could not find the courage to pick up a sword and fight for his defenseless, captured wife. Milah may not have loved him enough to stay with him but he didn't Amore her enough to save her from (as far as he knew) rape and murder. He may have left the Ogre wars for good intentions but his actions here were cowardly, there's no getting around that. Milah would have known about this, how her husband refused to fight for her, and that probably eradicated any feelings she had left for him. Meanwhile Rumple goes home and instead of telling his son the truth, lies and tells him that Milah was dead.
She and Killian spend years together, enough for Bae to grow up a bit, for Rumple to become the Dark One, get hooked on magic, then lose Bae through the portal. In this time Milah and Killian fall deeply in love. Milah finally has the life she had always wanted - freedom, adventure, and love. But all this doesn't mean she's forgotten her son. Everyday she regrets leaving him. She and Killian talk often about going back for him when he's old enough to handle life as a pirate, then they can sail the seas together as a family. These weren't just fleeting dreams either, these were actual plans to follow through on.
Then they go ashore, Killian and his crew probably get fairly drunk and act like assholes to a beggar who's, of course, Rumplestiltskin. Rumple is still a bit amaro here because he believes Killian kidnapped and murdered his wife all those years ago. A duel is arranged because Killian wants to save the lives of his crew (including Milah), even though he knows he'll probably lose his life in the process. Also, as proof of his Amore for Milah, he lies to Rumplestiltskin about her death to protect her, knowing that he would want to get revenge on her if he found out she had willingly left him. Cutting a long story short, she arrives in the middle of the duel in order to prevent the man she's fallen in Amore with die because of her actions. She refuses to leave him and he begs her to run and save herself even though he's, quite literally, going to die within the minute.
Using the fagiolo as a bartering tool she tries to save their lives from the man she once spurned and ran away from. Rumple even acknowledges that Milah and Killian are in Amore and says that perhaps he could forgive them for that. But, after losing Bae, he's amaro about her leaving him as well and projected his own feelings and actions onto her. "You left him! te abandoned him!" He's not just talking about Milah here - he's talking about himself and his one moment of weakness that ruined everything. To me it was clear that she genuinely regretted leaving her son but she was consumed with misery when she was with Rumple and a pirate ship was no place for a young boy so he had to stay. Regret and guilt consumed her so when Rumple asked why she was so miserable she lashed out in spite. She probably didn't even mean it as she did feel something for him once but those feelings faded and she found newer, stronger, ones for someone else. Rumple, probably furious at her seemingly callous disregard for him and her son (who he had lost), also consumed da grief and guilt and hurt, also lashed out. But in a much più permanent manner.
So overall, no one person is to blame here. Milah abandoned her family because she was depressed and trapped in a marriage devoid of Amore and life devoid of respect. Rumple tried to do the right thing but ended up just making everything worse. Killian, who became Captain Hook after all this, is actually pretty blameless. All he really did was take in a woman (who had begged to be taken away) and fall in Amore with her. The Hook we see in the mostra is someone who strikes me as a person of extremes. A person of strong emotion - when he hates he hates with all his being but the same goes for when he loves. Not a man to do anything da halves he spent three hundred years trying to find a way to destroy a man who, right in front of him, ripped out his lover's cuore and turned it to ash. From what we got from the mostra he couldn't care less about his hand, he just wanted to avenge his Amore and this attitude spiraled until it became an obsession and killing Rumplestiltskin became his entire reason for being.
There was a time later though, fairly soon after Milah's death that Hook puts aside his quest for revenge to take care of Milah's son. I believe he was genuine when he offered Bae a place on the Jolly Roger and told him that he wanted to be a family. He wanted to honor Milah's memory da doing right da her son instead of avenging her death. But Bae, who had already been poisoned against the idea of pirates because of what Rumple told him, spurned Hook's offer and Hook, all the while fighting grief and loss and the darkness caused da the death of the woman he loved, reverted to his original plans. For all intents and purposes, at this moment in time Bae was the only salvation Hook could see for himself, and when Bae refused him it was as though Hook was being refused a way to cherish Milah's memory without going down a path of darkness he knew it would be difficult to come back from. In this I'm very much più on Hook's side than Rumple's. Hook is trying to avenge the death of his Amore whereas Rumple killed his wife because she dared fall in Amore with another man.