Answer:
1. Roman dominion over the new lands became too large for a Republic to sustain. The Roman Republic was immensely successful in conquering new lands and bringing their political structure, culture, and religion into them. However, the Republic grew overwhelmed with accurately representing the territories with their representatives, and this became even more apparent at the end of its lifetime.
2. The lust of power/political corruption of the Senate and other representatives of the Republic. The Roman Republic relied on the virtue and ethics of the Senate, consuls, praetors, dictators, etc. The unspoken honor code of the Republic did sustain itself for hundreds of years, but it ultimately faded into nothing as representatives became more self-interested. Additionally, the rich patricians would bribe representatives with money to get what they wanted.
3. The Civil War between Marcus Licinius Crassus, Pompey the Great, and Julius Caesar. These three figures were longtime soldiers and political figures of the late Roman Republic. Like the rest of the corrupt politicians, they wanted to gain power to themselves. Crassus died in battle after forming an alliance with Pompey, so it left Caesar and Pompeii to be the 'last one standing.' Caesar famously declared Civil War by crossing the Roman border of the Rubicon. After Pompey was killed by the Egyptian King Ptolemy, Caesar was left to claim absolute authority over the crippled Roman Republic.