Star Trek movies portray Captain James T. Kirk (William Shatner) and the Klingons as bitter enemies trapped in a cycle of hatred, but Kirk and the Klingons didn't always hate each other in Star Trek: The Original Series. Kirk's hatred of Klingons is a key plot point in Star Trek VI: The Undiscovered Country. When the Klingon moon of Praxis is destroyed, Kirk would rather "let them die" instead of Starfleet rendering aid. General Chang (Christopher Plummer) also despises the Federation, so Chang uses Kirk's hatred to frame Kirk for the murder of Chancellor Gorkon (David Warner) and sabotage peace.
Earlier in the Star Trek timeline, Klingons are different. In Star Trek: The Original Series season 2, episode 26, "Errand of Mercy", Kor (John Colicos) is a Klingon leader who openly discusses the Klingon Empire's philosophy of conquest with Kirk, who respectfully disagrees. Later, Kang (Michael Ansara) in TOS season 3, episode 7, "Day of the Dove", doesn't attack the Enterprise unprovoked, but is tricked into believing Kirk is the aggressor. In TOS season 3, episode 13, "Elaan of Troyius", it's not Kirk that the Klingons are after, despite it seeming that way, but Elaan's (France Nuyen) dilithium crystal necklace.
Captain Kirk & Klingons Didn’t Hate Each Other In Star Trek: The Original Series
Kirk & The Klingons Respect Each Other As Leaders In TOS
It's important that Star Trek: The Original Series portrays Kirk as a thoughtful, evolved leader.
Captain Kirk's attitude towards the Klingons in Star Trek: The Original Series is a great example of series creator Gene Roddenberry's vision of humanity's future. Star Trek's humans have moved beyond prejudice, so it's important that Star Trek: The Original Series portrays Kirk as a thoughtful, evolved leader. Because Kirk can hold civilized discussions with anyone, even his enemies, the Klingons have a certain amount of respect for Captain Kirk as a skilled strategist. According to Roddenberry's vision, it's supposed to be hard for Kirk to succumb to hate—but it isn't impossible.