Final answer:
Marullus and Flavius are upset with the commoners due to their apparent ignorance of Roman history and values, along with the plebeians' participation in the moral decay and cultural corruption of Rome, as depicted in various historical Roman literature.
Step-by-step explanation:
Marullus and Flavius are upset with the commoners because they are disgusted with the lack of historical awareness and the disregard for Roman values that the commoners display. This sentiment is echoed in various sources from Roman literature that express disdain for the decline in Roman ethics and the rise in luxury, foreign influence, and corruption. These texts illustrate a time in Roman history where traditional Roman virtues like austerity, valor, and service to the state were perceived to be threatened by an influx of wealth, opulence, and the pursuit of personal ambition over collective well-being.
Similarly, historical figures like Marius undermined traditional military recruitment standards, signaling a decline in the meritocratic values once held dear. The corruption, assimilation of foreign practices, and the rot within senatorial ranks, noted by various senators and ordinary citizens alike, fueled the resentment of figures like Marullus and Flavius against the plebeians, who they saw as complicit in the overall moral decay of Rome.