This isn't to say the pair couldn't have been motivated by idealism as well as self preservation. They might certainly have believed that the Republic was a virtuous and worthy institution--after all, the idea of innate equality is relatively new, so the fact that the senate essentially bent to the will of the upper class might not have bothered them.
But I tend to think that they understood the senate was a corrupt, underhanded institution, full of bribery and B.S. From what I've read of Cicero's letters, I would say he certainly understood this, so why would Cassius or Brutus not understand it, too? To continue this line of thinking: If the pair knew the senate to be corrupt, they certainly couldn't have believed killing Caesar to protect it was noble--more likely, this was something they said--or even made themselves believe--in order to justify the act. I firmly believe the primary motivation was that the power of the corrupt senate was tied to their own personal interests, and Caesar with the masses threatened that.
Think about it: Who gets hurt if Caesar becomes an all-powerful king? Not the plebs, who are already basically subject to the whims of the senate (which is synonymous with the patrician class, really). If Caesar's reforms had, say, increased the patrician power rather than jeopardizing it, I don't think the senate would have plotted against him.
This is what I believe based on my understanding of Roman history.