Final answer:
The shift to violence in the Roman Republic was caused by external barbarian pressures and internal strife such as militarism, social inequality, and political corruption. This period of upheaval dramatically contradicted Roman values of order, thus being considered abominable.
Step-by-step explanation:
The sudden change to violence in the Roman Republic was precipitated by a variety of factors. The republic was marked by extreme militarism and was obsessed with war, which led to annual conquests and subsequent brutality, such as the destruction of the city of Veii.
As the empire grew, so did its challenges, including an increase in barbarian invasions, which were characterized by better organization and increased strength, leading to the eventual fall of the Western Roman Empire. This period of violence and transformation was starkly opposed to Roman ideals of civil behavior and order, which considered such actions as abominable.
Moreover, as Rome expanded, internal socio-economic issues magnified, such as the displacement and impoverishment of Roman soldiers, leading to social unrest and further challenging the stability of the republic.
The essential issue leading to the decline and fall of the Republic was the combination of external pressures from growing Barbarian forces and internal strife, including social inequality, corruption, and the misuse of power, a notion conveyed by Cicero in his speeches against the violence and corruption that were undermining the republic.