It was a combination of many things of which two are: (1) the barbarians absorbed Roman military tactics and devised ways to defeat them and (2) an endless series of succession battles - including an era that saw 20 emperors in 60 years - exhausted Roman manpower and finances. Despite its decline and fall, Rome was significant for being one of the few empires to last over 1000 years. Its wars of succession were bloody, but these wars ensured that the new leader was, first and foremost, a military leader.
A combination of effects with a single cause which was an immoral agency acting on the individuals in the society which led to the things you describe.
I’d point out that the plague around 300 effected the Empires enemies too.