Would the slide into amorality of the Republican Party have happened regardless of Trump ?

I think so. It's been going that way since Reagan. Check out the documentary "Boogie Man". It tells the story of Lee Atwater, the father of bare knuckle Republican election propaganda. He's the guy who fabricated the apocryphal "Welfare Queen."
Watch Ava Duvernay's documentary 13. Slavery never went away. They just created the 13th amendment to enslave Black people permanently. I know the Dems own some of that, but the GOP are the racist party.