Any isekai with a harem is instantly shit and this dogshit fucking terribly written trash exemplifies why.
The premise is super interesting, assassin who has done nothing but assassinates gets reincarnated with a time-limit in order to assassinate the hero. Great, that's a setup with shittons of potential. Watching him grow, watching him acclimate to the new world, watching him, watching him react to actual warmth from an actual family, etc.
What does this piece of shit anime do? It teases you with a cool old man assassin who never shows up after episode 2 in character or in person. It completely showers OP with effortless victory after effortless victory. It gives him two broken girls who in the span of literally 10 seconds go from "I want to die, life is misery, my backstory is nothing but tragedy" to "okay random boy who showed me a tiny bit of kindness, I'm going to be a mindless assassin/fucktoy 4u without any reservation or character, I only exist to stick knives in people and wanting you to stick your dick in me".
Oh MC's just figured out how to create superalloy metals, levitate, reverse gravity and create guns, I wonder how that's going to affect the world in the slightest? Lmao it doesn't it just gives the MCs blue knives.
Oh the MC's house has a jealous older rival who thinks very little of the MC, I wonder where is conflict is going? LOL MC just wipes the floor with rival and then offers the most patronizing shit up and the rival just eats it up and is now loyal, there's that thread gone.
Oh the MC's going undercover as a merchant in order to learn more about society and build a long con identity for later use, this will be a very interesting look into his methods and his character. Fuck all of that we're just going to gloss over it all!
Fuck harems. Fuck isekai. Most manga is dogshit at worldbuilding and characters and I'm sick of it because they at least come up with somewhat interesting premises.