In Dragon Ball Super, the sequel to the iconic anime Dragon Ball Z, the main character Goku faces a challenge unlike any he’s ever faced before. Long story short, Goku, his rival Vegeta, and the rest of the z-fighters have a friend from a different timeline, where all of the z-fighters died, including Goku. The friend is named Trunks, and he’s an older, alternate timeline version of Vegeta’s son. Future Trunks is the lone hero of his timeline, but when Goku lived, he was the beacon of hope to those around him. This is where Zamasu comes in.
Zamasu lives in the same timeline as Future Trunks, and is the apprentice of a god, but he fell down a dark path. He also has the power to essentially shapeshift into any human form. He chooses to appear the same as Goku, making him Goku’s doppelganger.
Zamasu, now known as “Goku Black”, terrorizes the timeline, abusing his appearance as Goku, a symbol of hope and strength. When Goku and Vegeta hear about this, they hop across dimensions to come to Trunks’ aid. What ensues is a clash of good vs. evil, and hope vs. despair. The combined forces of Goku, Vegeta, and Trunks struggle to defeat Goku Black, but after episodes of fighting through adversity, they defeat him, and Trunks lands the final blow.
It was important for Trunks to land the killing shot on Zamasu, because it shows that a new figure has stepped into Goku’s role in that timeline. Trunks has lived in a world that had a desperate need for hope, and ultimately he was the one that was able to provide it.
For Goku, out of his numerous battles, this one might have been the most meaningful to him. Not only did he help prop up a new hero for people that desperately needed one, but by fighting himself, he was able to defeat some of his own personal demons, while taking down a villain that tried to masquerade as him.