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

Possibly. The frustration of repeated policy failures by the GOP would eventually push them to take more and more desperate measures. Sadly, even now that frustration looms, with 2 failed attempts to repeal the ACA shows that they haven't been able to write new policy. Add to that an unpopular tax bill on the horizon, and the dismantling of Net Neutrality, the GOP's agenda is looking more and more hostile to common citizens.