It would improve the society

No. It would create a society in which the powerful have decided which genes are the ones they like when in reality, its diversity and complexity that is what's 'best' for humans.

and people quality of life

That is not how eugenics works.

I don't understand why is it bad

Then take a biodiversity and ethics class.

Just to Godwin the thread, eugenics is the province of nazi's and other totalitarian regimes. Even though it could never work in the way they want, all anyone who advocates eugenics really wants is to eradicate other people different than them.