Goku joined forces with Piccolo to confront Raditz when this Saiyan invader first invaded Earth.
Goku is the central character of Dragon Ball. Despite being a hero in the eyes of many, he has often made questionable decisions. One of the most surprising choices was his decision to end Raditz, Goku's own brother.

According to many, although Goku doesn't seem to regret the departure of his brother, upon deeper reflection, this could be one of his life's greatest mistakes.
Firstly, it's important to remember that both Goku and Raditz are sons of Bardock, a low-class Saiyan warrior who showed great potential. Bardock always hoped his children would excel and become great protectors. Goku achieved this, but what about Raditz?
In some ways, Raditz is a victim of circumstance. Upon arriving on Earth, Raditz sought out his lost younger brother, known as 'Kakarot,' who is none other than Goku. Upon meeting his brother, he revealed Goku's origins and urged him to join the Saiyan army with Nappa and Vegeta. Despite his misguided and arrogant actions, Raditz was attempting to reconnect Goku with their Saiyan heritage. In the end, he lost his own life in the process.
It's saddening that Goku always shows mercy towards his enemies, from Vegeta to Frieza, but Raditz is an exception. While Goku gave Vegeta a second chance, helping the Saiyan prince turn over a new leaf and become a stalwart defender of Earth, with Raditz, Goku chose to sacrifice himself to hold Raditz in place so Piccolo could deliver the finishing blow to both.
Imagine if Goku had done the same for Raditz as he did for Vegeta, the story might have taken a completely different direction, with the two brothers uniting as Bardock always wished.
The Return of Raditz in Dragon Ball Super: Is it Possible?
Dragon Ball Super has provided answers to many questions, one of which is shedding light on Raditz's fate. In Chapter 83, we catch a glimpse of Bardock's past as he fights to protect Planet Cereal and wishes for the future of his children.

This only fuels the idea that Raditz has untapped potential, perhaps destined for greatness, much like his younger brother. After all, in Dragon Ball, death is never the absolute end. Raditz could very well be revived by the Dragon Balls. Who knows, in another world in the future, Goku might have the chance to face Raditz once more?

Raditz's departure is like a stain on Goku's mercy. With all the revelations in Dragon Ball Super, now seems like a good time to question whether Goku's decision to end Raditz's life was his greatest mistake? And perhaps more importantly, is there still time to rectify this error?