Interesting topic... for discussion from what I understand and I am no historian the Roman empire fell because it over extended itself by undertaking military conquests to colonize other countries.

Very soon it was running out money and borrowing from other to sustain its operations. The army was extended beyond its means and the empire became vulnerable to attacks from outsiders.

It was the greed of Rome and its citizens that led to its fall which you can say was a loss of its morals.