What ended the roman republic?
You could loosely say that the physical act that ended the republic was the murder of Julius Caesar. However the republic had been in decline since before the dictatorship of Sulla. The senate was split into two broad groups, those who wanted things done "the way of the ancestors" and were resistant to change, and those who were more progressive and realized that times and conditions had evolved from the early days so some changes were necessary. It was this disunity in the senate that effectively ended the republic. As one writer put it, the republic fell because of grumpy old men.