Both the Senate and certain individuals are responsible for the collapse of the Roman Republic. They would not allow any of the soldiers or poor people to have any of their land or money because they were too worried about it. The main individual that is responsible is Julius Caesar because he had one thing to achieve and that was to be the dictator of all of Rome. Sulla was also one of the main individuals that is responsible because he had to use his army as force to get what he wanted. There are many reasons why they are both responsible.
The Senate did many things to help the collapse of the Roman Republic. They would not carry out any reform measures that the people wanted because they didn't want to have to give up anything that was theirs. They would not give up any more land to the people in the army and who were poor and did not have a nice place to live.
They also lost all of the soldiers' loyalty because of the land that they would not give them. When Tiberius wanted to give the poor people land, he skipped the senate and took it straight to the Plebian Assembly. This act by him made the senate upset so they killed Tiberius and 300 of the people who supported him. This act by the senate made everyone upset. The senate were worried about upholding traditions that they have had for a long time like taking a law to the senate first then taking it to the Plebian Assembly (Tiberius broke this tradition). When they killed Gaius, they weakened the government. The senate broke up in the evening after Caesar made his proposal. All who belonged to that order were summoned by Pompey.