Why did Godzilla not kill Kong?

Godzilla let Kong live in the new movie, Godzilla vs. Kong, because they formed an alliance to battle and defeat Mechagodzilla.

It was Godzilla’s intention to kill the massive gorilla in order to reassert his status as the King of the Monsters. However, the circumstances of their clashes, as well as how Kong helped Godzilla destroy Mechagodzilla, changed his mind. Their truce left earth with two alpha Titans, one for the surface world and one to rule the Hollow Earth.

The movie takes place three years after Godzilla: King of the Monsters. In that film, King Ghidorah was awakened from his millennia-long Antarctic slumber, which then set off the rising of over a dozen other Titans around the world. The creatures were summoned by Ghidorah, who set out to become their ruler and destroyer of earth. It is known that Ghidorah awoke Godzilla’s rival, Mechagodzilla.

Godzilla ultimately killed Ghidorah and established himself as the King of Monsters. The only Titan who did not submit was Kong, ruler of Skull Island, but the Monarch built a containment dome over Skull Island.

In Godzilla vs Kong, Monarch breaks the containment of Skull Island when they join with Apex Cybernetics to transport Kong to Antarctica. The plan is the use Kong as a guide to lead them to the Hollow Earth. This where Apex hopes to harness the ancient energy hat powers all of the Titans. However, Kong is vulnerable to an attack from Godzilla and sure enough, the lizard goes after his main rival. Kong nearly drowns as he has a huge disadvantage in the deep sea. Thinking Kong has submitted, Godzilla leaves Kong alive and departed.

Shortly after, Kong and Godzilla have a rematch in Hong Kong, when the lizard realizes that the ape found his ancestors ancient throne room in the Hollow Earth. Godzilla stomps on the ape’s heart as Kong drops his axe. This is what signals his tapping out to Godzilla. The lizard leaves Kong to die.

The Mechagodzilla shortly makes an appearance and Godzilla realizes he is completely overpowered by his robotic doppelganger. Kong soon comes to help Godzilla and they combine their powers to completely destroy Mechagodzilla and end the threat of Ghidorah once and for all.