The fall/decline of Rome is an incredibly complicated, long, and confusing event. Though the many instances of Imperial infighting is one answer, there are many others.

If anything, the 'take-away' idea from Rome and her history is that you can 'take away' anything you want. Rome was many things at many times. The thesis is that there is no thesis.

Are you saying that the Roman Empire, was at any point, lead by someone self-sacrificing and honest who was focusing on the greater good?

Because that’s really not what I took away from classical history.