Goku and Krillin first became friends when the two began their training under Master Roshi at the same time. While overly competitive at first, Krillin and Goku became best friends while they helped each other get stronger and stronger through their shared training. Before long, both fighters proved themselves skilled enough to enter a world martial arts tournament where they each went incredibly far but ultimately failed at winning the overall event. While losing their first tournament was painful, the pain of not winning a competition didn’t compare to the tragedy of the second tournament they entered together, as that one ended with Krillin’s mysterious death.
In Dragon Ball chapter 134 by Akira Toriyama, the Tenkaichi Budōkai has ended and Goku finds the lifeless corpse of Krillin after he had been mysteriously assassinated by an unknown assailant. After Goku races off in the assassin’s presumed direction, Roshi reveals that he knows who killed Krillin: King Piccolo. King Piccolo was an evil Namekian who tried to take over the world centuries before the events of this chapter, but he was trapped in a mystical containment vessel to keep his evil at bay. This chapter reveals that King Piccolo had escaped his containment and his first villainous act was to assassinate all of the greatest martial artists in the world starting with those who participated in the World Martial Arts Tournament. His first victim happened to be Krillin.
Using Krillin’s death as rageful inspiration, Goku eventually killed King Piccolo by shooting his body through the villain’s torso, brutally ending his life. While that moment was epic, it wouldn’t be the last time Goku would use Krillin’s death to inspire him to defeat an incredibly powerful threat. When the Z Fighters heroically traveled to Planet Namek many years after Goku killed King Piccolo, Krillin’s life was ended once more, only this time the assassin was the universal tyrant Lord Frieza. Frieza killed Krillin right in front of Goku as a way to prove his awesome power and to make Goku too fearful to challenge him.
However, Frieza’s kill had the opposite effect on the Saiyan warrior as the rage he felt due to Krillin’s death gave him the boost he needed to unlock Super Saiyan for the first time, and in that form, Goku soundly defeated Frieza. While Goku had a few victories before and even in between Krillin’s two deaths, those were of little consequence to the wider Dragon Ball mythos. Goku beat low-level villains like the Carrotter, Emperor Pilaf, and even the Red Ribbon Army with ease without the need for inspiration from the death of a close friend.
However, when Goku went up against Lord Frieza and King Piccolo, he may not have survived those encounters without an extra push—and Krillin’s death helped Goku unlock the power he needed to defeat those Dragon Ball villains in both instances and win two of his greatest victories.