The Evil Organisation that the great elder had entered an agreement with were not only a threat to ordinary people, but all cultivators and since they had gained so much power from the shadows, they were even more dangerous.
They had underhanded means to get the negative energy that they needed, such as through hospitals. The energy in the area around a specific area was affected by the living beings and plants in the area and was also affected by nature and the elements themselves.
This meant that when one was going through trauma or feeling negative emotions, they would unknowingly emit small amounts of negative energy, which would accumulate and was beneficial to cultivators of negative energy.
In a society filled with mental health issues and many different ways to negatively affect someone, a cultivator of negative energy would have an abundant amount of negative energy within a densely populated city.