but if we are a bit more observant we can notice that Minjee allows it out of revenge, since MC reproached him for his cheating and let him know that she doesn't share his vision of a partner.
You mean that Daeshin reproached Minjee for cheating on him? He said to her after getting beated up by Nelson that after they get out of the city they are stranger to each others so Minjee dont caring about him after that and even leaving him behind does sounds like a pitiful female's self justification of personal justice/retribution which does explain why she doesnt feel guilty(tho she avoids eye contact with you, either because she doesnt stand your presence or she has some guilt) but in the end only mades Minjee an asshole.
But if Daeshin kept appearences and didnt say that to Minjee, does his situation could have been ended up being better? Keep in mind that by the second time Nelson fucks Minjee she is already contemplating dumping Daeshin even if is in a joking manner.
Minjee does refer to Nelson as sir so she does respect Nelson at least for being reliable in the context of an apocalypsis.
Judging by she calling Nelson daddy in one of the sex scenes and also mentioning a father in Jiwoo's route i have to guess she is kinda of a daddy's girl on top of being kinda childish, i mean she wanted to become an idol. I wouldnt be surprise if her reasons for wanting to follow that path are childish as well or too foolish(maybe foolish is the right word).
In the Old man's route, despite Minjee being fucked a lot by him she doesnt fall for him and she even apologizes to Daeshin afterward, in the bandit's route she enjoys herself but also she is terrified and really doesnt like the men thst sre fucking her beyond just the sex so she is looking more than just sex skills, it seems that Nelson is also very good at talking and seducing women, you can see how much Minjee mentions that she likes to heard him talk.
In the end maybe wasnt that far-fetched all things consider, including Minjee's own reasons to justify not feeling guilty for Daeshin and deciding to leave him behind when she escaped with Nelson in the car.
This deserves another route were Daeshin kills Nelson succesfully just to know how Minjee would react to it.