Of all the species and races in the Dragon Ball multiverse, the Saiyans, of which protagonist Goku is a member, are obviously the most prominent. Known for their power and fighting spirit, the Saiyans were some of the fiercest conquerors in the universe before being almost entirely wiped out.
Their trademark rage and emotionalism is their most renowned trait, but it may also be their greatest weakness. In fact, several elements of the anime and manga showcase how power and niceness in Saiyans may go hand in hand. It's possible that the once highly feared race could have reached even greater heights of power and conquest if they simply kept their anger in check.
Rage of the Saiyans
The Saiyan race, being a species of bred warriors, are many times portrayed as drawing strength from their anger and fury. This boosts their raw power, helping to launch them into primal rages of violent aggression. Unfortunately, the Saiyans are known more for their ferocity than their intellect or even battle tactics. In fact, the prevailing fan theory is that this warrior's madness actually keeps the majority of Saiyans from achieving their true physical peak.
The other side of this theory is that the most powerful Saiyans are the calmest and nicest of their race. This can be seen in Goku and his sons, whose heroic demeanors allowed them to achieve powerful transformations such as the Super Saiyan form and its various upgrades. Goku was even able to achieve a Super Saiyan 3 form, which caused Vegeta, who lacked the transformation, to become angrily jealous. Vegeta himself is also far more confrontational and less calm than Goku, which backs the theory and explains why he would be unable to turn into a Super Saiyan 3.
Saiyans Could Be Calm Conquerors
Other bits of potential evidence that back up this theory include the fate of Nappa, as well as Goku and Vegeta's descendants in Dragon Ball GT. The former was even more callous and perturbed than Vegeta, yet he never showed even a fraction of the other Saiyans' potential before being permanently killed off. Likewise, in the final episode of the now non-canon Dragon Ball GT, Goku and Vegeta's descendants very easily become Super Saiyans at an incredibly young age, doing so as simply as any other action. The calmness and lack of brazen fighting spirit that accompanies this transformation says a lot about how much stronger the Saiyan bloodline has become by abandoning their original culture.
Goku's strongest form yet, the Ultra Instinct form, is achieved by letting go of all emotional blockage and simply folding into a serene state of being. This is perhaps the biggest evidence that Saiyans' rage could be their biggest hindrance. Perhaps by following something similar to Goku's methods of more disciplined but reserved training, as opposed to relying upon mere brute force, the Saiyan race as a whole could have fared better against enemies such as Frieza.
Another example to follow would be that of the Nameks. The warrior class of these slug-like aliens makes them essentially warrior monks. These beings have a stronger focus on meditating and holistic training than the savage Saiyans were known to employ, resulting in immensely powerful beings such as Kami/Piccolo. Even Nail showed incredible power before he fused with Piccolo, lending credence to the idea that Namekians produce more impressive warriors than the mediocre bruisers of the Saiyans ever did. While this has never been confirmed outright in any media, the indication is certainly there.
