A boy falls in love with a girl and this girl happens to be the demon lord's daughter. Unable to confess to the demon princess as a lowly human subject in a demon ruled lands he determines himself to one day become a person worthy of her.

He himself is a reincarnated soul from earth with no noteworthy besides retaining his old memories but he finds himself getting involved with your typical isekai hero types when one day one suddenly falls out of the sky in front of him. Fully aware that letting this guy go will likely result in the loss of his girl one way or another, he kills the would be hero and takes his cheat sword for himself. 

Believing that this deed and newfound if incomplete power would merit him a chance with girl, he attempts to convince the demon lord to let him have his daughter's hand in marriage. The demon lord however still deems him unworthy and dismisses his warnings of more heroes being summoned to replace the failed first attempt. It is only by his daughter's prodding that he relents and tells the boy that if everything he said was true then he'll reconsider his worth if he were to eliminate the remaining 8 isekai heroes that will be summoned to their world.

He manages to find and kill the second hero in a battle that nearly lost him everything but in doing so, news spreads that someone has killed a hero. With the rest of the heroes likely to be on alert from that incident, he decides to pose as the first hero and bring them down without doing a repeat of what happened with second one as the true heroes will only be honing their cheats further from this point while he'll only ever be able to use fragments of their powers. 

Thus begins the protagonist's harrowing quest to decide how his future and his world's future will be shaped by the choices he makes and the consequences they bring with them.

>Has encounters with the other heroes and easily fools them because of his past memories
>Demons also attack them while they gather powerups and only a few know about MC's true identity
>Demon empire isn't entirely bad for humans and there are worse deals outside but demon lord is set on conquering more lands
>Princess keeps in contact via memory/soul powers and animal familiars
>They discover secret prophecies foretell a total of 10 heroes are supposed to be sent to this world but 10th never makes themselves known to them so they figure they're dead too
>Kills half of them when he gets the chance or found out
>One is a love interest, one colludes with him for a cushy life, last two are rivals one of whom dies and passes their power to the other 
>Colluder gets two killed including love interest and rival kills him in disgust 
>Rival is suspects MC through the story but trust hims after killing colluder
>By end MC has 6/9 fragmented cheats while rival has 3/9 full cheats
>They move to take on the DL but MC takes rival to a trap
>MC convinces rival to submit using hostages as per DL's orders
>MC finally allowed to marry princess but knows he could have fought for a better future for everyone
>Princess reveals she was the 10th/original hero all along but gave away her past memories to MC when they were kids since they tore her up inside since she loves her family in spite of their tyranny
>MC's memory communications with her troubled her too and one last memory share of the journey pushes them to act
>MC and princess free rival and they take on DL together
>Rival sacrifices himself to give MC the full set of cheats
>DL is defeated but MC is crippled and will die young
>Princess fulfills her role as the last living hero and last royal demon to make sure their sacrifices weren't in vain
>MC is celebrated as a true hero. MC gets to see their efforts fruits before he dies