Moral values that had for centuries kept the Roman army, and therefore the empire, together, fell apart towards the end of the Empire. Complacency and ill-discipline among the legions sowed the seeds for a lack of structure that ultimately weakened the Empire. What was once tight military might had become a bloated rabble ripe for the taking. Meanwhile, violent crimes made the streets of the larger cities unsafe, while such Emperors as Nero and Caligula became infamous for decadent parties that wasted the Empire's money.
Historians believe that sanitary conditions actually played a significant role. Many of the wealthy had water brought to their homes through lead pipes. This of course proved toxic and as a result, the death rate amongst the affluent was very high. Then there was the continuous bloodshed at the ever-popular Colosseum. The great many gladiator conflicts brought not only death and violence but also disease. It's also worth mentioning that alcohol use increased, adding to the general incompetence of the general public!
Most Romans were not rich. Rather, they lived in small smelly rooms in apartment houses with six or more stories called islands, with each island covering an entire city block. At one point in its history, there were 44,000 apartment houses within the city walls of Rome. These apartments were hot, dirty, crowed, and dangerous.
For the countless poor, this was their only option. They rented their homes from the wealthy few, but anyone who could not pay the rent was forced to move out and live on the crime-infested streets. Many could not afford even to live in these squalid conditions and over time, the rise of the homeless, combined with the appalling living conditions, caused the cities throughout the Roman Empire to decay.
It's important to remember that the trade which bound the Empire together was by and large agriculture. Over the years, however, farming became a dog eat dog industry. During the final years of the Empire, farming was done on large estates called latifundia. Those owned by the wealthier landowners survived as they were run using slave labour. As a result, any farmer who was actually paying their staff could not compete.
This highly competitive market not only undermined the average citizen farmer who relied on passing his values to his family, but also filled the cities with unemployed people. At one time, the Emperor was importing grain to feed more than 100,000 people in Rome alone. These people became a burden to the Empire, as well as contributing to an ever-increasing crime rate.
When it came to choosing a new Emperor, the Roman Empire never established a smooth and efficient system of election. The choice was always open to debate between the old Emperor, the Senate, the Praetorian Guard (the Emperor's private army) and the army.
Over time, the Praetorian Guard gained complete authority to choose who the new Emperor would be, who in turn rewarded the guard with even more powers. This continued until 186 CE when the practice of selling the throne to the highest bidder began. During the next 100 years, Rome had 37 different emperors - 25 of whom were removed from office by assassination. This ultimately undermined the stability and strength of the Empire as a whole.
During the last 400 years of the Empire, the scientific achievements of the Romans were limited almost entirely to engineering and the organisation of public services. Basically they were so busy trying to maintain the welfare of their sprawling, bloated population that they didn't try to keep on top of what was going on around them. More specifically, Rome had stopped conquering other civilisations and adapting their technology, and was actually losing territory it could no longer maintain. As a result, the Empire became an easy target to potential enemies.
Maintaining an army to defend the borders of the Empire from barbarian attacks was becoming far too expensive. This strain on the Roman economy led to many other areas being under-financed, such as providing public housing and maintaining quality roads and aqueducts. Everyday Romans thus lost their desire to defend the Empire. The Empire had also begun to hire soldiers recruited from the unemployed city mobs, leading to an army that was unreliable, potentially treacherous and very, very expensive.
Many historians have argued that the massive cultural shift from paganism to Christianity was the final nail in the coffin for the Roman Empire. By adopting a belief in salvation through Jesus, the Roman culture as a whole became less interested in the here and now, and more concerned with the spiritual rewards in heaven.
Christianity also preached pacifism and turning the other cheek, resulting in a less aggressive attitude to potential enemies such as the Germanic armies from the north. In the third century CE, Germanic forces began to overtake Roman lands in Greece and Gaul. In 476 CE, the Germanic general Odoacer overthrew the last of the Roman Emperors, Augustulus Romulus, and the once proud Roman Empire in Western Europe soon crumbled into dust.