The main character is a thief/scout/assassin added (along with his Precious Childhood Friend, a mage) by the queen to an existing 3-person Hero Party before they depart on their winding mission to defeat the demon king. The MC immediately, on first sight, makes the hero for a True Believer naïf who can't possibly understand the obviously evil(?) things they'll need to do along the way. He confirms this by asking if the hero believes in his mission, and when the hero says yes, the MC decides on his own that he must assume certain burdens and leap into inaction.
They're underfunded, being ripped off by merchants, and under attack from spies and assassins both within and without the kingdom. The MC decides, on his own, that knowing about any of this would be detrimental to the Hero Party's 'purity,' basically. What's this mean in a practical sense? No idea, honestly. They'd give up their mission if they knew the obvious, not so awful truth? They can't be trusted to continue to be a good public face for the kingdom if they're aware of its seedy underbelly? They'd leave the problems unaddressed until they were killed by them? It's never made explicitly clear. If you're thinking "huh, 'pragmatic darkness' and 'idealistic purity?' that sounds a lot more like how an edgy child thinks about things than a morally gray, cynical adult," then put a pin in that thought.
So here's where things get really, really baffling: The MC decides to address this issue by personally handling whatever profitable crimes, coercion, extortion, torture, extrajudicial killings in the crowded streets, etc. that he deems "necessary" while very much an obvious part of the hero party. Yep, while keeping his allies totally in the dark, dude blazes a trail of Jack Bauer/James Bond highly-visible counter-espionage and black ops while standing out as the world's most UN-deniable asset. Would it shocked-pikachu you to learn that he gets super duper caught by both the public and his party?
So it's not just an accusation of "you're lying to us," or "you're doing Bad Things when you're one of the Good Guys!" that the Hero Party confronts him with, but also "dude you are absolutely tanking our reputation with the citizens, what the fuck?" They tell him that if he keeps it up they'll have to ask him to leave, and he only defends himself to them by saying that what he did was necessary. No explanation or anything. Note that they are not kicking him out, just warning him.
Naturally, his childhood friend, with whom he is in mutual but mutually-unspoken love, is heartbroken by his transformation into an apparent villain and unrepentant psychopath and his CONSTANT rejection of her attempts to reach out to him (for her own good, I'm sure.) When the hero tries to calm her during this confrontation by placing an arm around her, the MC gets jealous and goes full "you can't fire me, I quit," abandoning them with no explanation, only the words "hope you won't regret this," which, yes, obviously he hopes tragedy befalls them and they regret it.
They would be correct to fire a secretive and unaccountable loose cannon. The MC's whole approach was naive and stupid. He's really bad at his job. And let's be real, "If that girl can't deal with me ignoring her and pushing her away and becoming a homicidal maniac and thief, and doesn't have unconditional trust in me, and allows the nice and perfectly reasonable Hero who treats her decently to put an arm around her, THEN THE WORLD CAN JUST GET WIPED OUT BY THE DEMON KING, I'M TAKING MY BALL AND GOING HOME!!!" is not really a defensible position.
Anyway, in the second chapter, does the MC continue to do his job of being the dark force that looks over his love and the Hero Party from the shadows, this time ACTUALLY from the shadows?
lol, no, silly! He betrays her and goes back to the capital and marries the yandere in charge of the Dark Guild with an attitude of "sure, why not?" Not that we really care, because the mage was never really given any development as a character to begin with. His new wife is... devoted, if off-putting, and immediately puts him to work torturing and killing people for her clients.
Let's check back in with the childhood friend and her party: She's now covering the work he used to do in addition to her own, though not as well, because he never left any notes and she's 'naive.' They hire a new thief to manage their life savings, and that thief immediately runs off with it all. Things fall apart as they now need day jobs and one member has a breakdown after learning her father was killed and their noble household dissolved for corruption. The mage is finding solace over her heartbreak at the MC's betrayal in a budding or imagined romantic relationship with the Hero who's been emotionally supporting her since the MC left. She goes off to do her day job of adventuring and is immediately abducted in an alley and gang-raped and beaten for a day, which she explicitly reflects on as her "punishment" for "betraying" the MC.
She's not even dry from washing the vomit off herself before they receive a letter from the queen asking them to return, and that they're making peace with the demon kingdom.
Oh, and that female thief who stole their money is now naked in chains with the MC and his new wife, ready to be raped to death, begging for her life.