Outside of that poor choice, she was shown in the flashbacks to be the last one willing to go along with the rest of everyone's poorly advised, desperate plan to 'save' the MC from himself. She was clearly in agony and fought to change their mind, before finally acquiescing. I do think she failed the MC in the end, and for that she is punishing herself and not standing up for herself when the MC insults and is other mean & cruel to her. She feels like she deserves it and is not fighting it (within reason), even though her beloved is still trying to set her up with someone else (which is arguably just as hurtful) who she has no interest in and won't take a fucking hint to leave her alone.
In the end, Deb really should have insisted their mother confide in the MC about their family past sooner so he wouldn't have carried so much hate and self-loathing for his feelings. Additionally, if she was going to lean into having sex with multiple female partners, it probably would have been better for her to continue a side relationship within their family while making sure the MC was the only man she slept with so as to avoid these other women from getting too attached.
Can't you see the other side of things here? The things that went well for Deb? Strictly she shouldn't expect anything like marriage with mc; she knew exactly what her relationship was. She should have backed off from it.
But even knowing it could and would hurt her friend, she still kept fucking the mc. It's equally his fault too isn't it? Absolutely. Therefore as much as I would like to say she should have broken off this relationship, they both didn't. Having said that the mc's wife has him followed and catches them in the act. In my opinion the ex wife is worse than Deb but that's a whole different conversation.
So Deb and he have an agreement not to hold secrets from each other and BAM! As soon as she gets offered some hot pussy that promise goes right out the window. She said "he never knows or this stops here." She should have let it stop right there. And it keeps going. Deb didn't make some grandiose sacrifice; she's not a martyr, she's someone that would willingly risk ruining her brother's relationship by fucking the mc, and someone who would keep secrets behind the mc's back by fucking his wife. And the worst part is she pretends she did this FOR him implying his wife would have whored herself out if Deb wasn't the outlet.
The excuse: She did it because she "loves him" goes right out the window both ethically and literally. If a stalker saw a girl he loved struggle with debt and bills, kidnaps her and puts her in his basement, effectively taking away those darned debt collectors, she should be understanding that even when her life is ruined and destroyed how ultimately he did this out of love? It doesn't matter if he would take a bullet for her or not, he made a bad decision and should face condemnation not praise. Perhaps it's not a perfect analogy, but Deb made his life worse, not better.
She did what was ultimately convenient for herself, not the mc. And this idea that her mom forced her to ruin the mc's life? The characters in this story aren't 7 years old. No one but Deb herself can choose how far she watches him sink in misery, wasting years of his life chasing after what is taken from him.
Let's make it clear: she stood by watching his wife use the daughter to extort him into signing a divorce. And she would have allowed him to keep sinking further if his ex-wife hadn't "died." She knew he might never see them again. And between him and his family's delusional plot, she chose their plot. Whether she was first or last in relenting to his wife and mom's decision about what direction the mc's life should go in is irrelevant.
Do you think any proper husband/father would ever say "I would prefer personal safety over having my wife and child in my life?" No. So why did his family make that decision for him? Why are we expected to feel that what they did was even remotely forgivable?
To me at least: Deb is someone who struggled, and failed, against her inner demons. She had to pick between her love and baser needs, and just like the mc and his wife, she failed. But for whatever reason the mc is the only one who gets any blame for it.