Pei Xuanjing's unrestrained killing spree in the Blessed Land of the Forbidden Place was not out of any whim, but because he had already reached this point, and next he had to clearly define his own path.
A Third-grade Martial Artist is also known as a Grandmaster.
A Grandmaster is one who pioneers a school of thought and sets up principles that others look up to as a model.
The Upper Three Realms are not just about increasing strength recklessly, it's more about opening up your own path, leading a school and setting up a model.
During this process, some people follow their hearts, but they can't stay true to them, and are gradually corrupted by power, ultimately falling into a state where they can't control their power and instead become tools of it.