You're being awfully insensitive there, when someone asked you to stop something, so you double-down on the behaviour.
The
crisis of the third century was the most likely core culprit for the later fall. It massively eroded central authority, and lead to much more localism. Local lords began to ignore tax collectors, since enforcement collapsed. The city-based economy and the middle class declined, with economic power shifting to land-owning noble families who accumulated vast tracts of land by force during the 50 years of civil war (thus sowing the seeds of feudalism) because food was the one sure resource to keep value. These nobles then had monopolies on farm production, and used this power to massively slash agricultural output to keep prices high (these were local monopolies as long distance transport of food would not have been practical).
So you had all economic power shifting to people who slashed production in the interests of personal greed. Obviously ongoing high food prices and shortages aren't doing much for your country militarily. I doubt the later christian thing had any measurable impact. The idea that Christianity made them too peaceful thus they got invaded (as seen in some of the links) is a dubious claim given that one of the main problems was that the many Romans factions were too busy trying to
kill each other rather than properly deal with the barbarian invasions in the early 5th century.