Across the assorted Goku is generally portrayed as a friendly and helpful person just trying to be the best fighter in the universe, but he was not always that way. In his earliest years, he even rivaled Vegeta for brutality and cruelty.
As the saying goes, Goku has mellowed with age. His laid-back cheerfulness as an adult is most obviously illustrated by his knack for befriending his enemies and turning them into his most die-hard ers. Moreover, despite being commonly despised by his opponents, it's rare to observe Goku giving them the same hate back.
According to Dragon Ball Z, as a child Goku was downright Vegeta-like in his ruthlessness and brutality. Indeed, considering his youth, the friendliness that Goku exhibits as an adult is perhaps less the reduction of his having a pure heart, and more of a side-effect of his quest to be the best. That is, Goku helps and is friendly with others because it is in some way helpful to his goal to become the ultimate fighter.
The main source of his youthful savageness is the fact that his whole reason for coming to the Earth is to infiltrate it, kill all its inhabitants, and sell off what was left. Moreover, he was sent to do this on his own as a three-year old child. This fate was only avoided by the head injury Goku suffered when his ship crashed. That head injury damaged his memory and his ability to recall his mission, and dampened his inherent super-savage side. But that mission is an ever-constant element that will always have to potential to make him less friendly, even as an adult.
Despite the head injury, kid Goku still proved to be an aggressive, destructive, and uncontrollable force of nature throughout the original Dragon Ball manga. Perhaps the most shocking result of this dark side is his killing of Grandpa Gohan. While this is an accident, it nevertheless shows the raw, unrestrained savagery that Goku can exhibit. There is also his mass extermination of soldiers of the Red Ribbon Armies. Indeed, even though they are just doing their jobs, Goku seemingly kills them with neither remorse nor guilt. Subsequently, Goku is responsible for the deaths of Octopapa, Ninja Murasaki, Captain Yellow, Tambourine, Drum, and, most impressively, King Piccolo, to name a few. Goku’s kill rate and kill list as a child are significantly greater than it is when he becomes an adult. To be sure, as an adult the only people Goku is known to have killed so far are Yakon and Kid Buu.
With such a dramatic change between Goku as a child, and the adult version, the question becomes what could have caused such a drastic change. One popular fan theory is that after the original “bump” on Goku’s head that caused him to forget his original mission, each time he died and was resurrected, his inherent savageness was pushed ever deeper into his consciousness. This has allowed the human proclivity towards justice that he learned from Grandpa Gohan to become dominant.
Another theory argues that Goku’s savage essence has never really left him and that it lies just under the surface waiting for the right circumstances to “go live.” Lastly, there is the idea that as he has gotten older and been able to reflect on his life experiences, he has concluded that savagery simply does not help achieve one’s his outside of the ring. Whatever the reason, it is clear that as and adult, Goku better fits the definition of a Dragon Ball hero. Whether he will be able to maintain this iteration or will revert back to his original more savage self, remains to be seen.
