Aside from his most recent exhibition of power during his latest Dragon Ball appearance in the film Dragon Ball Super: Super Hero, Piccolo has pretty much been put on the sidelines compared to the other heroes and villains he was once on-par with, which is a far cry from his early days in Dragon Ball where even the gods of their world secretly considered Piccolo a bigger threat than even Frieza.
Upon his introduction, Piccolo was not a Dragon Ball hero–in fact, he was one of the cruelest villains in the history of the franchise. Piccolo began his career as a being who considered himself a literal demon, King Piccolo, who’s one and only goal was to throw the world into chaos and inspire atrocious acts of violence in every citizen. Before he was able to take over the world for any significant amount of time, Goku kills King Piccolo, but before the villain dies, he produces an egg containing his reincarnated form–the version of Piccolo fans know and love today. While the reincarnated Piccolo completely redeemed himself for the evil actions of his previous life, he evidently didn’t earn everyone’s trust by the time Frieza came around.
In Dragon Ball chapter 314 by Akira Toriyama, Goku and Frieza are in the middle of their iconic fight on Planet Namek. Meanwhile, Piccolo is standing off in the distance with Krillin and Gohan, monitoring the fight and being ready to enter if it seemed like Goku needed him to. In this chapter, Goku is losing pretty terribly to Frieza, and the Saiyan decides that the only move he has left to play is the Spirit Bomb. When Goku begins this technique, Piccolo is stunned as he has no idea what Goku is doing. It is only until Krillin tells Piccolo that King Kai taught Goku that move during their last training session that Piccolo finds out about the existence of the Spirit Bomb. Piccolo even curses King Kai at this moment for showing Goku and not him despite the fact that Piccolo was just training with King Kai, saying, “He never said anything about that to me…”.
King Kai is a god over an entire sector of planets in the Dragon Ball universe, and he has observed Earth for a very long time. Because of this, King Kai knows exactly how destructive Piccolo can be if he wanted to, which is most likely why King Kai decided not to share with him this specific technique as it has the potential to destroy an entire planet without draining the user of hardly any power since Spirit Bombs pull from the ki of those around the person creating it rather than just that person alone. However, Piccolo was just training with King Kai for the sole purpose of becoming powerful enough to kill Frieza, so it seems like King Kai really should have shown Piccolo everything he knew to ensure that Frieza would be defeated–but the fact that he didn’t indicates that King Kai is secretly more afraid of Piccolo’s potential than Frieza’s current threat level.
It makes sense why King Kai wouldn’t want to help make Piccolo more powerful than he has to, given the specifics of Piccolo’s past. Not only was Piccolo a villain who believed himself to be a literal demon, he was also the incarnation of every ounce of evil expelled from the body of Earth’s God, Kami. So, Piccolo wasn’t just a Namekian who went bad, but the concentrated negative byproduct of a mortal reaching the level of a god, and King Kai–being a god himself–knew exactly how dangerous something like that truly is. Frieza is a murderous tyrant and a scum of the universe to be sure, but King Kai still saw Piccolo as a much bigger Dragon Ball threat–and the fact that he didn’t teach him the Spirit Bomb when he really should have proves it.