How did socialism contribute to the decline of the Roman Empire?

In order to continue it's social/socialist programs in the capital city and empire, Rome would enable massive taxation to it's municipalities and territories (land whose residents would not be afforded the rights of a Roman Citizen), as well as impose trade restrictions ( a good example was Rome's price fixing of wheat in order to support it's burgeoning welfare system - Roman citizens were "entitled to the right" of "free" wheat, the "Annona") to keep artificial prices on certain commodities. This lead to massive inflation, which in turn put an added burden on the Roman state.

Rome would continue to conquer in order to obtain resources to fuel it's insatiable appetite.

Socialism is a grand idea on paper, until you put it to practice. Then it reveals itself as the parasite it is (it consumes everything to keep the ball rolling).