that's how we call it a story. In this part, Rocks don't have the will and strength to fight back because Garp and Roger fought him until they collapse.
Rocks character was so great that his writing is good and well valued, the reason for him to get killed is to make 'Propaganda' from holy knights and the Government itself. It's like an important part of the main story on why Rocks was meant to die—it's already written since chapters ago about it that he would due but we don't know his character back then. Now that we view what kind of character he is—that's where another plot will come to play. :))
too goated for the fraud world
Why Oda have to make Rocks so great then kill him??