Bad analogy (as usual) there bud. Hitman is a +15 year old franchise, it's bald assassin and social stealth mechanics are iconic and influential. You can't just do the same and pretend it's not a ripoff.
On the other hand let's see Alex...
>If somebody makes a game
Alex never made a game (In his own words, there is no win state so nobody can criticize it as a game). Yandere school is a finished product with multiple languages like portuguese.
>Yandere schoolgirl
Ayano might be "iconic" via sheer internet fame but she isn't a yandere and Alex didn't invent them.
>killing people in a school setting for love
That's common in anime and games, even outside of yandere stories lol. Yandere school actually has some original ideas like killing Senpai's male friends instead of just a single female rival.
>Stealth/action
Yansim isn't stealth/action, it's more like a puzzling mess that makes no sense. Yandere Schol has a much simple but pratical stealth gameplay, you find a weapon-> find opportunity-> kill-> hide body-> GG. It's ironically more close to hitman than yansim is.
tl;dr: Not a "ripoff". Alex had the idea first but they were the ones who actually made a game with it.