When Goku defeated Frieza on Planet Namek, he not only eliminated one of the greatest threats to the universe, but he also reached a whole new level of power in the process–though after one particularly enlightening battle in Dragon Ball Super, it seems as though that victory should have never occurred.
Goku defeated Frieza during the climactic battle of Dragon Ball Z's Frieza Saga, one which began after Frieza decided to hunt down the Dragon Balls of Planet Namek in order to use their magic to become immortal. During his quest, Frieza laid waste to Namek, destroying everything and everyone who stood in his way. The worst part was that Frieza did so without any semblance of a warrior's honor. He was sneaky, slippery, and not afraid to play dirty to defeat the Z Fighters and even Goku himself. When Goku won the battle between them, he gave Frieza a fraction of his own energy so that he could have a chance at living through the planet’s destruction. However, Frieza decided to use it to shoot a ki blast at Goku when his back was turned, but his deceit was his ultimate downfall as Goku countered the blast with a finishing move that seemingly killed Frieza right then and there.
Frieza's Love of Trickery Should've Assured His Victory
In Dragon Ball Super chapter 10 by Akira Toriyama and Toyotarou, Goku and the rest of the Z Fighters enter a multiversal tournament between their universe (Universe 7) and a handful of warriors from Universe 6. When Goku steps into the ring, he is face-to-face with a fighter who is the spitting image of Frieza, and while on the surface he seemed pleasant enough, he was hiding a despicable secret up his sleeve that would have ensured his victory in the tournament if it weren't discovered. Even though weapons were forbidden, Frieza’s doppelgänger, a villain known as Frost, had a poison needle that was strong enough to incapacitate Goku with a single prick.
In Dragon Ball Super’s Tournament of Power, Frieza proves to be even more cunning than Frost by tricking him into an elimination. Based on that logic, Frieza should have thought of Frost’s poison needle trick long ago, and if he had access to it, should have used it during his battle with Goku on Planet Namek. Frieza is a prideful fighter who looks down on all other creatures, especially Saiyans, so it is no surprise that he'd resort to trickery in order to win right away. In other words, although he'd want to beat them with just his strength to prove his superiority, he also has a lot to lose here, so he'd want a backup plan.
Frieza Has Changed In Dragon Ball Super's Later Chapters
In chapter 87 of Super, Frieza makes a dramatic return and reveals that he has actually been training, and has achieved a new form known as Black Frieza. He proves the superiority of this form with ease, laying waste to Goku and Vegeta, even in their Ultra Instinct and Ultra Ego forms. While Frieza is certainly not a good guy, he proves that he has changed through his willingness to train and improve himself rather than relying on cheap tricks like Frost's needle to win. In that sense, he's actually learned the value of training from his nemesis. By actually being the strongest, Frieza can satisfy both his ego and his evil plans, knowing that he can handle even his most hated enemies with little effort.
It's clear that Frieza’s pride clouded his mind during his battle with Goku, but it is hard to imagine that someone so cunning and with so much to lose wouldn’t have an ace in the hole. Since he and Frost are nearly identical, it would make sense that Frieza would come up with a similar scheme to defeat any foe in an instant, just as Frost had done in Dragon Ball Super, proving that Goku never should have won against Frieza during their iconic battle on Namek.