Spock and McCoy in “Unfinished Business”

Spock

We have here an unusual opportunity to appraise the human mind, or to examine, in Earth terms, the roles of good and evil in a man-- his negative side, which you call hostility, lust, violence, and his positive side, which Earth people express as compassion, love, tenderness.

McCoy

Are you aware it's the captain's guts you're analyzing?

Spock

Yes, and what makes one man an exceptional leader? We see indications that it's his negative side which makes him strong, that his evil side, controlled and disciplined, is vital to his strength. Your negative side removed from you, the power of command begins to elude you.