What made the roman republic fall?

There were several reasons for the fall of the republic, but the main one was corruption. The senate was divided into two groups, the optimates, who were stubbornly entrenched in the old ways and adverse to change, and the populists, who advocated change in order to keep up with the times. Each group bribed their way into authority so they could put forth their own agendas and further their own concerns. The good of the state took second place to personal advancement.