I'm brand new to the forums here, so a hearty hello from Toronto. Being removed from the scholarly life for quite some time, I will come off a tad rusty....anyhow my comment on the fall of the Empire...
Vespasion's comment on who is culpable for the fall of the Empire appears to have a bit too much of hindsight. As it is true that Julius Caesar was the first emperor, you can trace the apprehension of later emperors to the first Caear and how you predicate the basis for his power.
However, unified peoples of antiquity are much like modern corporations: they can never have enough. Especially true of Rome during the Caesars, health in the economy was based in large part (if not totally), on expansion. As a Republic, Rome fared quite well in expanding her borders, but would have reached (in my opinion) either diminishing returns or a bottleneck if allowed to exist for another hundred years, to be convenient.
The Senate, as pointed out before, did a poor job in facilitating the machinery of government and its intention: to represent the populace. Let me make this distinction from popular rule. Being represented by the elected is one thing, but when the elected have it in their vested interest to keep from the populace, you end up with Senators being used as Roman candles.
Back to my original comment. I would not put the fault on Julius Caesar for having begun the end of the Empire. It was up until the end of the second century that saw Rome's greatest triumphs, with Trajan and Claudius in my opinon making the greatest strides for expansion.
However if I had to put one name next to the title of "destroyer of the empire" I'd have to name Diocletian, for a couple of reasons.
First, Diocletian is credited with the tetrarchy system of rule. This was the seed of civil conflict in the Empire, no matter how much is said that the Empire grew too large to maintain (forget ruling over) by one person. It has been my contention ever since I got a proper foothold in Roman history that this separation would manifest itself in the end of the Western Empire.
Second, Diocletian made himself more a king than emperor. As I remember reading of past emperors "ascending to the purple" wearing a robe of that colour, Diocletian made it a point to garnish himself in jewels. Furthermore, the commoners had to partake in more selfless actions when coming in contact with Diocletian, soon making the moniker 'princeps' a title fit for the Julio-Claudians.
As the tetrarchy would separate Rome into East and West (Constantine's sons would later separate West further), Diocletian's reform of imperial character would further separate Rome into rich and poor. Not that Rome before then was an egalitarian utopia. The average Roman, before Diocletian, would refer to the Emperor as a man among them. Diocletian effectively put an end to that, in ways even the most average citizen could see or hear of it.
A little long in the tooth, but submitted for your approval.