Why did Harry spare Pettigrew but wanted to kill Bellatrix?

In Prisoner of Azkaban, Harry forbids Sirius and Remus from killing Pettigrew by arguing that his father James would not have wanted his two best friends to become killers to avenge his death. Yet, in Order of the Phoenix, Harry chases after Bellatrix, threatening to kill her for killing Sirius. Why is there a discrepancy?