So from some research on Caesar's death and what I found said that the roman leaders thought he was making a bid to become king so they organized a conspiracy aiming to remove him and keep the system the same as it was. But his death lead to the Roman Republics collapse so and the Roman empire forming so it didn't even achieve what they wanted. On top of that I don't think that any of the assassins wanted to help the republic in anyway they just wanted to hold on to the power and Caesar being crowned would take that power away from them. So I think it was an excuse to keep power to themselves and in the end destroyed the republic.
Take all this with a grain of salt I only did some quick research so I might of gotten some facts wrong and I wasn't there and will never know the real reason why they killed him.