Ok, first off, I don't have a problem with the basic idea behind the premise necessarily. I've played and enjoyed Saya no Uta for crying out loud. I think horrifying premises can be interesting and powerful if done right. And that definitely includes this premise. But...
That "done right" thing is the kicker. There are certain premises, themes, and ideas that are tremendously difficult to pull off right, that take an extremely talented and sensitive writer to succeed. And your premise is definitely one of them. I just mentioned I enjoyed Saya no Uta, but I feel there are parts of it that are, to put it mildly, uh, problematic. I have a lot of issues with the game. (I wonder what it would be like if Gen Urobuchi wrote it today, with all his experience now?) I think there are some pretty masterful elements to it that let me enjoy it despite its flaws, but oh boy does that game have issues.
And these same premises and themes that I'm talking about also tend to turn out very, very badly if not done properly. It's a huge risk and you can potentially alienate and/or offend a whole bunch of people you never wanted to. It's one of the reasons why I, myself, will not currently write a game or story that features rape. And I don't just mean rape by a protagonist, I mean any rape. I just don't think I currently have the talent or knowledge needed to be able to write about it properly, in a respectful and sensitive manner. And I DO NOT want to screw something like that up. Of course that's just me, and there are certainly people out there who can write about rape, and do so properly.
The point is, I'd advise caution, and that you seriously ask yourself if you think you can pull off this premise, at least as right now. I don't mean to be discouraging, I just feel you should be confident about this before you start putting in serious work on this title.
There are a lot of potential pitfalls you can fall into. One is that villain protagonists can sometimes make it seem as though the narrative is justifying the decisions of the protagonist. This is especially true for visual novels I feel, where the element of choice and the fact that you are usually able to see inside the protagonist's head is really good at letting you empathize with them. In most cases this is a good thing, but sometimes it can be hard to sort out "is this just the perspective of the character, their rationalizations and reasons, or is this also the perspective of the game itself?"
And make no mistake, what you have described is essentially a villain protagonist, except for in one route that you described very briefly and which honestly sounded kind of boring. "You can choose not to do any of the three over two months and die. Game over. The end."
I'm going to talk about Saya no Uta in more depth, and include some major spoilers. But I feel it illustrates a point I want to make.
But there are some important things to note. One, the protagonist isn't the sole protagonist. His former best friend is a major character, and as the game goes on we start seeing more and more scenes from his perspective, till he's essentially a co-protagonist by the end. Heck, one of the game's only two choices is actually made for him. This, combined with some subtleties in the writing, allows the game to make it very clear that the initial main character is not really justified in what he's doing. It's understandable that he is doing (most) of what he is, considering the situation he's in and the madness and stress he's under, but the game clearly views his actions as terrible despite that.
These scenes where we focus on the MC's former best friend are also nice as they are a break from being inside the MC's head. The MC's perspective is so warped and twisted, especially as the game goes on, that reading his thoughts can make one feel a bit unclean after a while.
Another thing to note is that the first choice does not occur till halfway through the game, so early on as we watch the MC become more and more cruel and go deeper and deeper down the wrong path, we as the players are not actively guiding him that way. We have no choice but to watch it happen, helpless. By the time we do have a choice, the MC has already unknowingly done terrible things and gotten so far into the madness that there's no real way to save him. He can't make things right anymore. In addition, this is a slow process, and we spend a lot of time in the MC's head watching him gradually get worse and worse at the same time as we grow to empathize with him and his situation.
And another, potentially harder thing to do is that you will have to get the players to actively choose to have the MC commit murder. Well, start planning it anyways. I think part of the reason some people are reacting so badly to your premise is that the MC will be presented with a choice right off the bat to start planning to eventually murder someone, or kill himself/let himself die. In order to really play the game and progress in the developed routes, players are immediately asked to make the choice to kill.
And there's no build up to this, no gradual corruption and desperation, no agonizing or desperation. Just murder, or not. And furthermore, this will happen before we've had time to get to know the protagonist. And furthermore, the main aspect of his motivation that you've described is his experiencing death. But that is a visceral motivator, by which I mean that no matter how talented a writer you are, it is impossible to truly convey what experiencing actual death would be like, and no one reading it will be able to truly imagine what it would be like. And so the main motivator for our main character is something ultimately beyond our comprehension.
I kind of feel hesitant about this next part, as I feel it can come off as somewhat arrogant to suggest things to "fix" an idea. But you seem to really want feedback, so one thing that I think could potentially help things would be if, at the start, the MC asks the Mysterious Man what happens if one of the women dies due to other reasons. (Which would definitely be possible considering the ill friend.) And the Mysterious Man says that if that happens, consider himself lucky, he'll count it as a freebie for killing.
And so the MC is not immediately forced to make the choice of killing or dying. He has a third option, try to marry/fuck and let fate decide if he lives or dies. His chances would be slim, but better than nothing right? Obviously he should ignore his ill friend since she's the most likely to die of other causes, and focus on his co-worker and boss for the marrying/sexing. Except, convincing someone to marry you in two months is kind of hard. Maybe he should actually try to marry his ill friend, since he could use her illness as a reason they should rush.
Of course, isn't all that kind of heartless? But what if he had some feelings for one of the women anyways, and this whole thing is just the push he needs to confess them?
This altered scenario presents the MC with difficult choices, just how far is he willing to go to live, how much will he manipulate, but does not immediately ask the player to choose to kill someone, does not immediately give us a MC who will murder to save himself. I say not immediately, because of course as the situation goes on, as we get to actually know the MC and the other characters and maybe care about them, the choice can come up again. Maybe the MC succeeds in marrying his co-worker and sleeping with his boss. But his ill friend is still alive, and the deadline is ticking away. And the MC who constantly reassured himself that he would never kill anyone, that he just did this on the off chance he'd get "lucky" and live, starts to have doubts. He starts to question himself. And then a situation where he can act presents itself...
Or maybe he sleeps with his co-worker and marries his ill friend, and he learns his boss is planning to do some reckless, dangerous activity. Does he try to talk her out of it? I mean, she's a grown woman, it's her choice. He can't force her. But if he would normally try to talk her out of it, but now is quiet, what does that mean for him? And what if she actually dies? Not trying to talk someone out of something reckless when you probably couldn't have convinced them anyways isn't really murder, right? Right?
Anyways, again, I hesitate to say "this will fix it!" cause it comes off arrogant, but I think immediately expecting players to start planning an eventual murder is going to be a big hurdle to overcome, and this is one way to sidestep that. It still allows the game to explore the "kill to save oneself" thing but doesn't force that upon players and the MC till there is time to get to know him and maybe empathize with him a bit more.
On another note, I feel you should make it extra clear that the MC won't just die if he fails, he'll suffer an eternal suffering fate worse than death, and that whoever he kills will not suffer such a fate.
Anyways, my 2.12 cents.