The fall of the republic was a long and complicated process which, I believe, cannot be traced to one specific event. Sure, the reforms of Marius change a lot, but they would not have been possible without a number of other events that either happened or were prevented. I mean, you could also argue that the failure of the Gracci could be blamed, or later, Pompey as he refused to compromise with Caesar and Sulla for his use of the misuse of the dictatorship. There are, of course, a great number of other events that could be mentioned.
In relation to this, I believe that we had a short discussion a while ago on the destruction of the Roman economy and that is yet another question where you can follow several events further and further back in time, without finding one specific point here it all went wrong.