Romeo is not necessarily denying responsibility for his choices or actions when he says, "O, I am fortune's fool!" (III.i.134). Here, Romeo is recognizing that he fell for temptation when he went into a fit of rage after Mercutio's death and ends up killing Tybalt. Before that point, Romeo had been doing his best to quash the argument between Tybalt and himself. Submitting to vengeance and emotional trauma after he sees his best friend die was enough to push him to extremes. In this quote, Romeo seems to realize the process and steps that led up to him killing Tybalt which weren't present in his rational mind at the time that he resorted to violence to solve the problem. Romeo realizes his weakness, his humanity, and his lack of control over everything in his life.