Which negative trait of some leaders is marked by a cynical view of human nature that condones opportunistic and unethical ways of manipulating people, putting results over principles?

The answer is "Machiavellianism".

Machiavellianism is a personality trait (often found in the leaders) in psychology and is part of the "Dark Triad" (including narcissism and psychopathy). This means that an individual when it comes to their own benefit ignores all the rules and uses every possible negative trait to get their interest.