Even though it seems like it would be no surprise that Dragon Ball history, did something heroic during the early days of his Dragon Ball career, it is actually very surprising given how viciously villainous Vegeta started out within the continuity of the series. However, it is important to differentiate ‘heroic’ with ‘good’ because what Vegeta did was still really evil, it just happened to benefit the group of earthbound warriors he would later call his family.
Vegeta was introduced in Dragon Ball chapter 204 but his history within Dragon Ball lore goes back decades before the events of that chapter. Vegeta was originally the prince of all Saiyans who worked for the villainous tyrant Frieza—traveling the cosmos and obliterating all life on select planets so that the newly-vacant worlds could be sold to the highest bidder. When Vegeta came to Earth, he had expected Goku, or Kakarot, to have already wiped out all life on the planet as was his duty when Goku was sent to Earth as a baby. However, Goku had no memory of his orders to decimate the planet after suffering a head injury when he was a child. Despite the memory loss, Vegeta still considered Kakarot to be a traitor to the Saiyan race and thereby condemned him to death—a sentence Vegeta was all-too happy to carry out. However, Vegeta didn’t try to kill Goku until after he murdered another Saiyan who disappointed him even more greatly.
In Dragon Ball chapter 227 by Akira Toriyama, Goku is gearing up to go head-to-head against Vegeta after he soundly defeated Nappa in battle. While Vegeta was shocked that Goku was able to beat Nappa so easily, especially given how thoroughly Nappa defeated the other Z Fighters, he wasn’t worried about his own well-being in the slightest as Vegeta was vastly more powerful than Nappa ever was. In fact, Vegeta proved his Saiyan superiority by punishing Nappa for losing to an ‘untrained’ fighter such as Goku. Rather than allowing Nappa to return to their base of operations, Vegeta decided he had no further use for the failed Saiyan and obliterated him with a single ki blast before turning his attention solely on Goku.
While Vegeta killing Nappa was unarguably villainous and mostly served as a way to intimidate his opponent, he actually did the heroes of Earth–and by extension, his future self–a huge favor by taking out this threat to humanity’s safety. When the Saiyans first came to Earth, Nappa obliterated an entire city with the wave of his hand before killing nearly every member of the Z Fighters himself (minus Yamcha who was killed by a Saibaman) and overall just showed zero restraint when it came to satiating his proverbial thirst for blood. If Vegeta allowed Nappa to recover, since he is a Saiyan, he would have become way stronger since Goku brought him so close to death. After that, there is no telling how powerful Nappa could have become if he kept pushing himself, which is a scary thought for anyone who has seen him fight.
Even though Vegeta proved to be a much more sinister threat than Nappa in these early chapters—even exhibiting traits akin to Frieza himself—he at least shows restraint and can be reasoned with, which is proven true after he works with the Z Fighters to kill Frieza just a few chapters after this one. Now, Vegeta is a full-fledged hero, though based on Nappa’s bloodthirsty nature upon his Dragon Ball debut, it is reasonably doubtful that he would have shared Vegeta’s redemptive journey—meaning that when Vegeta killed Nappa, he unwittingly engaged in his first act of heroism, even if it was technically despicably treacherous and incredibly dark.
