When Dragon Ball first began, the series was a mix of action and comedy. But as it progressed and reached the point of Dragon Ball Z, the series became far more focused on the action. But all throughout, there are minor moments when the series shifts tones and embraces a more horror-tinged feeling -- and it actually works really well. Nothing quite captures that potential as well as Cell's introduction, with the character in general being by far the most frightening single aspect of the franchise.
Before Cell, there are only a handful of scary moments and characters across Dragon Ball and Dragon Ball Z. Because of the nature of the franchise and how much importance it puts on people fighting to overcome the most deadly of opponents, the franchise hasn't often embraced horror. There have been minor story arcs in the anime that've dabbled in it, like the unsettling Cave of Darkness storyline from Dragon Ball or the building tension of the Z-Warriors training against visions of murderous Saiyans during the Saiyan Saga in Dragon Ball Z. Even the Garlic Jr. Saga has its moments, as we see the supporting cast corrupted into vampiric forms. These moments force the usually unflappable warriors to tense up and get serious, ratcheting up the drama in the process.
The only time the franchise took such a villain and put them front and center was with Cell. The introduction of Imperfect Cell is one of the most effective moments in the entirety of Dragon Ball Z. A creation of Doctor Gero, Cell consumes the life force of others -- via stabbing them with his prehensile tail and draining them, leaving nothing but clothing behind as he drinks their body -- to add to his already impressive well of power. By the time Piccolo confronts Cell for the first time, the villain had already consumed an entire city's worth of inhabitants. Men, women and children have all been killed by the creature and effectively eaten in the most frightening way possible, only adding to the power of a threat strong enough to endanger the entire galaxy if he's able to achieve his true form.
When Piccolo arrives, he'd just received one of his biggest power boosts in the entire franchise. Having finally merged with Kame, Piccolo has reached a level comparable to the Androids, who were wiping the floor with him and the entire rest of the Z-Warriors earlier. In any other saga, a power boost like this would be cause for celebration, but Piccolo's arrival is constructed in a way that's far more reminiscent of horror films than the bombastic reveals that happened with earlier villains. Piccolo observes the silent city, the only noise coming from the wind blowing refuse through the empty streets. The whole time, a hidden figure shadows moves through the city's shadows -- Imperfect Cell, who slowly approaches Piccolo, dragging a victim behind him.
This once sequence establishes Cell as a completely different kind of threat, one the heroes should actually be scared of instead of eager to challenge. His unique abilities and design are all introduced in short order, done in service of making a threat actually feel frightening. It makes Cell stand out for the rest of the saga, his darker impulses being treated with a more sadistic and terrifying bent than villains like the chaotic Buu or the impudent Frieza. When it wants to, Dragon Ball has it in itself to create genuinely frightening villains, who can bring natural tension to a franchise that often needs it. Sometimes, the overwhelming power levels can take things too far and make them too big to maintain the tension -- but moments like this can make things far more exciting and tense.