Dragon Ball Z is the Trope Codifier for modern shounen action series, including its villains. Most of them are Card-Carrying Villains who do things out of the heck of it, but not too evil. And there are some who were created evil and do some questionable things. There are the few who are so twisted that they are memorable for being so ungodly demented and cruel and lacking the morals/mercy of their fellow villains who are either horrified and can't do anything about it or actually admire the person due to their overwhelming power.
Frieza has more than earned the moniker for being the most evil villain of Dragon Ball Z. Not only was he racist, he was responsible for the extermination of the Saiyans (a horrific act not because of the Saiyans dying, as the Saiyans were mostly genocidal monsters too, but because the Saiyans were nothing but loyal to Frieza) since he feared he was going to be overthrown by a Super Saiyan. Second, during his invasion of Namek, he killed many civilians, including Dende's caretakers and brother, even after he got the Dragon Ball. Not only that, he slaughtered several warriors and tortured both Nail, Gohan (more chilling by the fact he had a Slasher Smile as he brutally massacred a five-year-old child), and Vegeta to near death, and even had Vegeta crying and begging Goku to stop Frieza - who Frieza then shot through the heart. As evidenced in the line above in the starter for this trope, he even freely admits that all the horrible things he did in stories were indeed true. He then blew up Dende, too, when he learned the boy could heal people. And he finally blew a hole through Piccolo, proceeded to kill Krillin (by blowing him up), and threatened Goku's son, which led to Goku becoming the very thing that he feared most. Heck, if Shin Budokai is anything to go by, he was also directly responsible for driving Broly Ax Crazy when he blew up Planet Vegeta without even realizing it.
Also, after Goku became a Super Saiyan and was beating Frieza, he STILL continued to taunt Goku by gloating about killing "the little bald guy" (Krillin). And lastly, when Goku gives an almost dead Frieza enough energy to escape the exploding planet, he uses that little bit of energy to attack Goku from behind.
And all of it was done willingly. He wasn't born to be a monster (like Cell or Majin Buu) or insane (like Broly and some of the other movie villains), at least, not at first glance. He was just an asshole who reveled in his power. He obviously had enough funds to live a life of permanent luxury, but instead, he founded a company whose business was finding populated worlds, depopulating them, and then selling them to the highest bidder.
What about the manner in which he killed the Saiyans? Sure, it was relatively quick and painless, but even so, let's put this in perspective to figure out just how petrifying he is. Fears of a legend are enough reason to keep a tight rein on the native child prince and to send one of his top guns to kill a strong soldier and his entire team. When the native king and his platoon, and then the angry soldier, who survived the hit, get tired of serving under this guy's thumb, he smites said king with a One-Hit Kill and then proceeds to execute the most devastating Finger Poke of Doom in history - wiping out thousands if not millions of loyal soldiers, including an entire race and planet of them, save for a few who weren't on home base at the time, with a giant sun bomb generated and released from one finger - while sitting in his hover-chair, laughing maniacally and telling his top lieutenants to come out and see the "beautiful fireworks". That thing came from Hell.
If you thought Frieza was terrible, wait till you meet one of his ancestral relations, Chilled, in the Gainax manga Episode of Bardock. Like Frieza, he goes around slaughtering populations of planets for his business. However, he also has shown himself to be even more ruthless than Frieza: For one thing, he murdered one of his own henchmen simply because the poor sap was unfortunate enough to stand in his way, period (and Chilled wasn't even in a bad mood, to boot). Then he starts terrorizing the population of Planet Plant all because one of the inhabitants became suspicious of his motives and beat said inhabitant's father up and nearly killed him just to gain the rejuvenation fluids.
Chilled wasn't much more than a schoolyard bully compared to Frieza. Chilled lacked any real restraint or desire to plan, and so wasted most of his cruelty on pettiness. Frieza took the long view and made sure to spread the suffering as wide and deep as possible.
There's also how he acts during fights. With the exception of King Vegeta and Bardock, who he simply killed on the spot due to time constraints, he tortures anyone brave (or stupid) enough to fight him. Nail (who got an arm RIPPED CLEANLY OFF), Krillin (who was savagely impaled by Frieza with his horns in second form), Gohan (yes, even a five-year-old child, by slowly crushing his skull and LAUGHING AT HIS SCREAMS), Piccolo (with whom Frieza played paintball with, just remove the "t"), Vegeta (who arguably got the worst of them all, ultimately leaving him crying in excruciating physical and mental agony), and Goku (the ONLY one able to fully return the favour) were all beaten up in extremely painful ways simply because Frieza was able to do so without fear of interference.
Cell, an android created by Dr. Gero, goes around towns "eating" people with his tail just to increase his powers, then absorbs Androids 17 and 18 which results in his going One-Winged Angel (with the last being disturbingly prolonged in order to give the latter absolute fear of being absorbed). And, though programmed to kill, he has a sadistic pleasure in it that is all his own doing: he starts "the Cell Games" just to hold off destroying Earth so that he can "see everyone's faces contort in terror" and states that he would've hunted every single person on Earth one by one if he won. He beats Gohan to near death and creates "Cell Juniors" to torture the other Z Fighters so that he can transform to his Superpowered Evil Side, which he succeeds by destroying Android 16 in front of him. Unlike most of the examples shown, he is the only villain who succeeds in killing Goku (although that wasn't what he intended, as it came about as a Heroic Sacrifice to save Earth from being destroyed by his self-destruct sequence).
This is justified, as he's inherited Frieza's knack for being a sadist in the genes along with Vegeta's pride.
Dr. Gero could be called one for being responsible for everything that goes down in the Cell Saga. Cell also implies that he may have intended for Cell to destroy existence. Also, in Dragon Ball GT, Gero apparently wanted to replace all life with artificial beings.
Tao Pai Pai is a Psycho for Hire who cares only about profit and ruthlessly and remorselessly wipes out anyone who gets in his way with his bare hands. After he murders Upa's father and defeats Goku in battle, he laughs at Upa and tells him that he's lucky that he's still alive, meaning that he has absolutely no problem killing children. One of his worst offenses has got to be when he forced a tailor to overwork on making an outfit for him in about three days...which he rewarded by killing him. When he returns to Karin's Tower to look for the one Dragon Ball he neglected to find, he grabs Upa and throws him against the tower (fortunately, he was saved by Goku). After his defeat, he was rebuilt into an Ax Crazy cyborg who wanted nothing more than the deaths of both Goku and Tenshinhan. He's arguably theKnight of Cerebus for the entire series. While the Z Fighters heavily outmatch him in Dragon Ball Z, he still has no redeeming qualities.
Then again, Tao Pai Pai even existing in Dragon Ball Online, a game that took place at least a hundred years after the Majin Buu Saga, as a member of the Red Pants Army would indicate that, at the very least, Tao Pai Pai had some redeemable enough traits to be revived by the Dragonballs (assuming he either didn't survive the Human Extinction Attack or didn't escape the planet when Kid Buu blew it up).
While Majin Buu had some redeeming traits in his imperfect forms (Petting the Dog several times in his "original" form), his final stage, Kid Buu, is an Omnicidal Maniac that just blows up planet after planet for the sake of it. It's implied that he's not actually stronger than Super Buu, but without the influence of the Kais, he has absolutely no restraint. He kills just for fun.
Kid Buu might get a pass, since, without absorbing someone who's REALLY good, like the DBZ universe's equivalent of God, he's mentally incapable of not being an Omnicidal Maniac.
Babidi also qualifies. Not only does he orchestrate the revival of Majin Buu, ultimately making him the one responsible for the cataclysmic amounts of death and destruction that occur later, but in addition, he kills off Spopovich and Yamu, who actually succeeded in their task for him, just because he didn't need them anymore (and then comments on how enjoyable their deaths were). Then he throws Dabura, his loyal right-hand man who came up with the plan that lead to Buu's revival, under the bus in the same fashion once Buu is revived. Not to mention the fact that he took great delight in having Buu systematically kill the people of Earth so he could lure out Piccolo, Goten, and Trunks all for the purpose of getting his petty revenge against them. And when he's asked to stop this by Goku, he then announces that he'll have more people killed just to spite him. Finally, he explodes the head of someone who gave him the names of Piccolo, Goten, and Trunks, just because he found the information to be useless. He also constantly threatens Majin Buu when the latter doesn't obey him, which later results in Majin Buu giving Babidi his karmic death. He's up there with Frieza in terms of just being an evil asshole that doesn't have the excuse of being a creation born evil or insane.
Bibidi, Babidi's father, is even worse. For one thing, he was the one who actually created Majin Buu, and he's also directly responsible for countless deaths within the universe even before Majin Buu was sealed on Earth, as well as driving the various Supreme Kais to near extinction. Afterwards, when Kid Buu becomes Fat Buu, he actually goes so far as to manipulate a pretty pure-hearted Majin Buu into annihilating everyone by promising him cake as a reward.
It tells you a lot about the guy that he would knowingly create a being incapable of seeing the universe as anything other than a shoot 'em up video game and with the power to win that "game".
Androids 17 and 18 from Trunks' Alternate Timeline(ESPECIALLY 17). Unlike the main ones, where they rarely hurt people and were just trying to live, these two were malicious, unstoppable Ax Crazy killing machines who took pleasure in all the killing and destruction they wrought. Android 17 was particularly Ax Crazy, hijacking a car and running people over just for the lulz, and Android 18 would murder people who were giving her clothes willingly. They also murdered all the Z Fighters. Needless to say, it was very satisfying when Trunks returned from the main timeline and curbstomped them with ease.
What made it all the more clear is "The Reason You Suck" Speech he gave them. The two of them loved the terror they put into their victims, how helpless they made them feel before killing them, making Trunks' Roaring Rampage of Revenge on them all the more fitting because he makes it a point to ensure they know just how powerless they are before him before he killed them so they'd feel the same way as their victims.
They do possess a Freudian Excuse, however: apparently, Dr. Gero rewired their brains so they would hate humanity as plans for world domination. Plus, even they (or at least 18) did have one Pet the Dog moment, even if minor: 18 did think a boy was cute, and her reaction to 17's killing him (which is never shown) indicated that she fully planned on actually letting the kid live, though she says that her upset reaction is more because "it makes her look bad in breaking her promise", rather than any feelings toward the kid.
Although the Kashvar, aside from Hoi, in Wrath of the Dragon, hasn't actually made an onscreen appearance (they only appeared in flashbacks), they certainly fitted the bill from the limited information supplied. The Kashvar are basically a race of warlocks who, in Tapions words, view themselves as the superior race and, as such, are perfectly willing to exterminate those who are not like them. Because of this, they also instigated a horrific war within the South Galaxy on Konats by resurrecting a phantom by the name of Hirudegarn to essentially commit genocide against the Konats. And it is also implied that even before Konats was liberated by Tapion, his brother and a Konats wizard by cutting Hirudegarn in half and sealing him within the two brothers, who themselves are sealed in a music box under their consent. They, for the most part, succeeded in causing the Konats to become an endangered species. Several tens of thousands of years later, Hoi, the last member of the race, also manipulates the Z-fighters into collecting the Dragon Balls to breaking the seal of Tapion, and it is also implied that he broke the seal on his brother's music box before arriving on Earth to unseal the bottom half, and then proceeded to murder the brother with the bottom half. Later, he has several people killed to feed Hirudegarn, and then destroys the planet.
An often overlooked example: the sniper Van Zant, who shot Buu's dog and later Mr. Satan. Really. In his first scene, he and his partner shoot an elderly couple dead, and he then revels about how awesome it feels to kill someone. We next see the two shooting people at random in a nearby town before going to Buu's house, which leads up to Van Zant shooting Buu's dog and Mr. Satan, triggering the release of Buu's evil side. All this for no reason other than the sheer thrill of killing people. What's worse is that he uses Buu's destruction spree as an excuse, reasoning that everyone will be killed eventually, and even going so far as to pretend that Buu killed his victims. Humans Are Bastards indeed. His death couldn't come quickly enough.
Even more horrific, their actions resulted in the creation of Super Buu and thus the return of Kid Buu, which means that they are indirectly responsible for literally everything Buu does afterward. Super Buu does a One-Hit Kill on the entire human race and then, as Kid Buu, destroys a bunch of planets, many of which were likely inhabited. And all this because two psychopaths decided it would be fun to shoot Buu's puppy. The worst part is that, at the time, Buu had reformed and posed no threat to anyone.
What makes them terrifying is that they are ''completely normal humans'' with no superpowers. All other complete monsters in this page are either aliens, androids, or other supernatural creatures who use ki-blasts and other super powers for evil. These two use normal guns to accomplish bad things. It is terrifying to see that two regular humans can be just as bad as intergalactic alien warlords or insane androids.
And why does he do all of this? He's reckoned that with Buu flying around basically laying waste to the planet, he's probably going to die soon, and he decided it'd be fun to try to take as many people with him as possible. That's his entire motivation. Probably a deliberate irony on Toriyama's part, then, that he ends up being the very reason Buu actually destroys the Earth.
Turles from the Tree of Might OVA. He crosses the Moral Event Horizon upon planting the aforementioned tree that would suck up all the energy from Earth. He then manages to establish himself as a Complete Monster when he forces Gohan's eyes open to see the moon and command Oozaru to go kill Goku. Turles himself even said he doesn't care if what he's doing is good or evil.
