The killing of Dr David Marcus by a Klingon on the Genesis Planet in 2285, when he threw himself at the warrior ordered to execute one of Kruge's prisoners.
David Marcus dies in Star Trek III: The Search for Spock. A scientist of Project Genesis and the son of James Kirk and Carol Marcus, he goes down to the Genesis Planet with Saavik and there admits that he used unstable protomatter to make the experiment work.
After the Grissom is destroyed, Kruge's men capture David, Saavik and the regenerated Spock. When Kirk arrives and stalls for time, Kruge orders one prisoner killed to prove his resolve. The Klingon moves towards Saavik, and David attacks him and is stabbed.
Saavik tells Kirk over the communicator, and he stumbles to the deck in grief. The loss feeds the hatred of Klingons that Kirk confesses in his log in Star Trek VI: The Undiscovered Country, where his words are used against him at trial.