but that got solved partially without him trying because it turns out his system gives him like a year of life just for killing one bottom-tier monster and then it gets solved completely without him trying because of a random encounter was like "hey thanks for doing this thing that you did with no effort, btw, I am actually a god and I fixed your supposedly unsolvable problem without you asking me to"
MC's temporary boss/high ranker/harem candidate tells him not to let anyone leave town because it might be too dangerous. Random low-tier noob says she wants to leave anyway. MC pulls out a gun and points it at her head and tells her he'll kill her if she leaves. Like seriously, out of nowhere. He could have just said, "hey, the gold-ranker says you can't leave because there are monsters roaming outside" but naw, straight to gun-in-face. Keep in mind this isn't some grimdark fantasy, this is a hyper-modern world where there's supposed to be rule of law. Our MC is a psychopath, but it gets worse.
Random bozo tries to stick up for the girl and say the MC is being unreasonable, which is what every halfway sane person in the room is probably also thinking. Not our MC! Our MC displays his genius thinking to himself "Huuuuh, why is this guy I can easily kill trying to stop me. Gosh, I know I'm not wrong so...I know! This random bozo must be doing this because she's pretty" and spends like a minute patting himself on the back that he isn't swayed by pretty faces like everyone else. Author makes it clear he's also proud his MC isn't, the chapter title is 100% for real titled "What's Being Kind to Women?".
We never get to find out what MC is going to do to them because, suddenly, for no reason at all, random mid-boss antagonist who has been harassing MC (and other people) jumps in to start a fight against MC, not because of the girl but because he wants to pick on MC. They look like they're going to get in a fight. Mid-boss is definitely stronger than him, he literally beat MC like yesterday and let him go because "you're so weak you're not worth killing." Which is a classic mid-boss line, except only about two chapters before the author established the only reason he's here is because he's uh, here to kill people he thinks are weak. Riiiiight.
MC doesn't back down, because I don't know, being a dumbass threatening some random girl with shooting her in the face so she doesn't endanger herself is more important than his own life, which is all so stupid I can barely breathe. Fortunately, they're suddenly interrupted by random bullsh*t and everybody forgets what they were doing. In the ensuing few days MC exerts his dominance by beating up sh*t-tier background characters with no names and doesn't let anyone leave.
Somehow, this makes people trusting and admiring of him, which is important because he's about to lead a squad into battle because his boss/harem candidate thought all that previous bullsh*t was genius work by him. A couple of chapters later he declares he's going to stay and defend the town from monsters despite being having a power rating of effectively zero when there are literally a thousand other super-powered high level humans running around. Tens of thousands of years of combat experience between all of them, and somehow MC is piping up with his sh*t-tier ideas and people are calling it genius. It also feels super weird because our MC has been relatively focused on himself and his sister so far, and this is the first hint that he gives a sh*t about anybody else, yet he's suddenly burning with self-sacrificial desire.
Not long after, it turns out this was a tremendously stupid idea (which more experienced people than him told him at least three times, but he ignored because he's dumb) and he's going to die, 100%, no take-backsies. Just then he gets a system notification: Would you like to get your get-out-of-jail-free card I didn't tell you about and you didn't know you had? He's like: "sure". And then the magic wish automatically determines how to get him out of the situation with no input from him, and kills millions of monsters with a nuke that only hits bad guys and instakills everyone but the boss, and saves the day. No, I'm not joking or exaggerating in the least.
But wait, it didn't kill the boss. Boss appears and says, haha, you humans are so dumb, Superman-Goku Macguffin the 3rd isn't coming to save you, Superman is a dumbo and is fighting my sekret clone. By the way, I'm going to kill all of you, especially you MC. Suddenly, harem candidate goes, "Wait boss monster, can you please give me 3 minutes to confess my love for MC? Pleaaase? Pretty pleeaaase?" Boss monster goes: "Okay, sounds fine with me, but don't doublecross me or I'm gonna like, kill you. Uh, I guess I'm gonna kill you anyway but I'll like really super duper kill you or something", and he goes to sit in the corner and jacks off for awhile.
Harem-candidates grabs MC, and then is like, "omgersh boss monster, you so dumb!" and then she pushes through MC through a portal while boss monster is distracted. Staring at the portal he goes, "How dare you lie to me!" "Curse you humans and your predictable yet inevitable betrayal!" "It's a good thing you were too dumb to take your own portal, now I'm going to kill you even harder!"
But then she goes "Wait, let me give a dying speech. Okay, still giving a dying speech. man, why am I not dead yet? Guess I'll speech some more. Also, it's sort of weird that I pushed MC through the portal and stayed behind but he's still here but oh well, if the author's not paying attention why should I? MC even though I met you like last week and we've literally never had a conversation, you are the most amazing man ever and I love you."
Boss monster: "That's great, I'm gonna kill you now."
POW! Suddenly the goddess our delightful MC met last week in yet another random encounter roll after feeding a squirrel who turned out to be a immortal god-beast who was the pet of the goddess back in the day so he was like, how about I show you her corpse and then just because MC is MC the goddess came back to life from him getting a plot boner or something and was like "Oh hey MC, we're soulmates and I love you. Please ignore my irrelevant background story which makes it seem like I should vengefully kill all humans." Anyway. Goddess shows up and her pet squirrel smacks boss monster to death with a single slap. Actual Deus ex Machina. It's not every day you get a god to solve your unsolvable problems. Unless you're the MC. Anyhow, Goddess says, "BTW MC, since we killed a immortal super boss monster, why don't you take its soul. You can raise it as a soul pet and get super powers you didn't work for, again, just like the time you became a grandmaster in a sword technique that takes a lifetime of effort to learn by watching me make tea."
MC: "Huuuh? Oh, okay. Oh yeah, goddess, can you help save this harem candidate who I met last week?" "long as you're here doing miracles, y'know?"
Goddess: "No, that is impossible... unless I had a boss monster soul or something like that, but those are pretty rare and it's not like we just have those lying around." "Oh whoa, it turns out there's a boss monster soul I just gave you. But are you sure you want to give it up? You worked so hard for it. Oh wait, no you didn't I just showed up and gave it to you."
MC: "uh, sure, yeah, whatever G-dog"
Goddess: "haha, just playing, I don't need the monster soul after all. It'd be too much of a pain in the ass to write another monster in...I mean, I don't need one because I got others. Despite me being dead for the last thousand years or whatever" "Bippity-boppity-boo! This girl is alive! But hey we're still gonna get it on at some unspecified later date when you're a god through remarkable happenstance, right?" "also, the girl is super mega powerful now cuz I put a god-beast in her. The soul I gave her was like a super rare Phoenix soul I just had lying around."
What drives the plot is the conceit that classes impel a person to fulfill being the ideal representation of their class, and the stronger the class, the stronger the compulsion becomes. This idea is examined in-depth and is the motif running through virtually every story arc.
Also, probably plot-centric because the Hero happens to be a yandere bro-con, and the MC is the onii-san in question. And for the record, the reason she's a yandere is also explained in-universe making her a pitiable Hero.