I think the timeline is as short as it is here for gameplay purposes.
I agree, but even narratively it doesn't seem as if they've spent
that long together. The progress they make together is more the work of days, not weeks or months. And as presented, their own task-oriented behavior and her more standoffish nature mean they're not really forging enduring connections during that time.
If the narrative made a stronger effort to imply they
have been together longer (with "Three Months Later" sort of captions or montage sequences or whatever, or just the implication that they've been having sex multiple times over and not just a single night of intimacy before the hammer drops), or if the bad guys didn't reveal Selene's collusion immediately (prolonging the period of time he trusts her for), it would be more justifiable.
But even then, love can easily turn to hate when you realize that you've basically been betrayed and manipulated from the very beginning. That's when "Was any of it ever really real?" and "How can I ever trust you again?" feelings start to kick in, and the hurt you feel from that sort of thing can very easily spiral out of control into very dark places.
Just from the story as presented, it's entirely believable that the MC could be more willing to lash out at her or refuse to forgive her for what she's done rather than just passively suck it up and pretend like it never happened. The latter is sort of shading into what a lot of people complain about when they whine about doormat MCs in a lot of VN games.
About the other thing, the MC killing the terrorists (or whatever they are), I don't get that vibe from him at all, he would at least never ever harm Selene (and her sister).
Not as presented, no. But how he's presented is what some people are complaining about. At least some people probably feel like he should be at least willing to consider it.
A lot of it also depends on how you view his perception of things. WE the players know that Selene is telling the truth about her sister, but from the MCs perspective he has zero reason to believe her at all. She has just spent days lying to his face, manipulating him to get what she wants, and generally playing him for a fool. Why should he assume that
anything she says is true at this point?
As far as his knowledge of the situation extends, it's entirely possible she lied to him about her entire childhood (to garner sympathy), lied about her sister, and is lying about being forced to trick him. She may be fully committed to the evil plan and simply manipulating him to keep him in line.
And beyond that, even if he
does believe her, there's always the fact that he can still be pissed because she's still the reason he's in this mess, and why his family is at risk. She's still participating in the evil plan,
even if coerced.
Humans aren't rational creatures, and he has plenty of reason to be both rationally and irrationally angry with her. But as of yet the
player isn't really given the option to express that in any meaningful way.
The MC doesn't really have a reason to consider Selene an enemy, if he quit his "holier than thou" routine for a second he would realize that Selene had no choice here, since her sister is in the hands of the villains.
It's not really "holier than thou" to have the attitude that someone who is directly responsible for your current problems should be held accountable for that, even if they were only acting under duress. Because there are
always other alternatives, and "I only did it to protect my loved ones" only goes so far as justification when you are placing other people at risk.
And even beyond the "The Needs of the Many Outweigh the Needs of the Few" utilitarian argument, there's also the "Are You Really That Stupid?" argument - does Selene really think any of this ends with a Happily Ever After for her and her sister if the evil plan is fully carried out as intended? The most likely scenario is that the villains are going to torpedo the colony ship directly into the planet if they turn it around and send it back to Earth (or at least crash it into the moon where everyone on Earth can see), so everyone on board is going to die (fanatics tend to be fine with suicide missions, after all). And even if the plan is meant to end with the bad guys escaping, there's no real motivation for them to leave anyone else alive - in fact, killing Selene and her sister before they go would be a pretty smart tactical move on their part to cover their tracks.
And that's assuming you believe her at all, and don't assume literally everything she says is a lie and she's happily part of the plan. Which at the moment, the MC would be entirely justified believing (again, we the players know,
but the MC doesn't). He has zero evidence or proof to suggest her version of events is true, and a fair amount to suggest she's lying.
Just shrugging and going "Well, you've lied to me constantly so far, but I'm going to believe you now and forgive you for everything" kind of makes him feel a bit impotent. Doubly so when, no matter
how things pan out, her actions have still effectively destroyed his entire life and placed
his loved ones at risk. That's not all that easily forgiven under the best possible circumstances.