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

I would think so. Historically, when there are periods of economic uncertainty, political groups lean towards fascism as a way to provide order, or at least the illusion of order. Within the Republican Party (and it's not every Republican, but certainly different ideological factions) there is this militant need for stability.

Disregarding marginalized groups, ignoring environmental concerns, being callous to the needs of the downtrodden, etc. is all done to preserve this ideal. Republicans have traditionally had a narrow worldview, but when facing pressure for results, the worldview tends to become narrower.