Political corruption was widespread throughout the Roman Empire but particularly in Rome and within the upper ranks of the Praetorian Guard. The superiority of the Praetorian Guard, which consisted of the most distinguished and decorated soldiers within the empire and personal bodyguards and counsel to the Emperor, led to the majority of the political corruption in the empire and grew to such an extent that the Praetorian Guard held secret meetings to conspire to overthrow the Emperor and to decide on who they saw as a suitable replacement. When the Emperor Tiberius was overthrown the Praetorian Guard auctioned the throne to the highest bidder for 100 years. Political corruption also led to many civil conflicts within the Roman Empire. The Romans failed to develop a suitable system to determine who the new emperor would be, unlike the Ancient Greeks before them who held elections. The decisions were made by the Senate, the Praetorian Guard and the army but in the end the Praetorian Guard had complete authority to choose the mew emperor who was often the person who offered the best reward to the Guard. For the majority of the 3rd century the title of the Emperor changed 37 times with 25 of them being removed by assassination and this contributed to the overall weaknesses, decline and the fall of the empire.
There is no definitive reason as to why the Roman Empire collapsed, if there was to be one defining factor I believe that the over-expenditure on military expansion as it put strain on the Roman government as it cost them many economic and military casualties as they were losing a load of money hiring soldiers to invade and often replace them when they died without gaining from the lands they conquered. In my opinion, this is the major factor pertaining to the collapse of the Roman Empire. The political corruption allowed the Praetorian Guard to be above the law and announce whoever they wanted as Emperor regardless of whether they were capable of the task. The dependence on slave labour caused high unemployment and the stagnation of technology for the last 400 years of the Roman Empire. The Roman Empire is said to have completely collapsed when the German barbarians overthrew the last Emperor, Romulus Augustus in 476 and introduced a more democratic form of government which was very short-lived.