A good villain can make any work of fiction exciting, and some of them aren't even truly villainous or evil. Some of them are merely worthwhile antagonists who may be doing the right thing but are still at odds with the protagonist. Ideally, a worthwhile villain or antagonist in an anime series will stick around for a while to get some solid character development and contribute to the plot across many scenes.
However, some anime villains or antagonists die much sooner than fans would like, even if the villains' death happened at an appropriate time in the story (such as when their character arc is complete or they lose a fair fight). Some villains could have been deepened even more or show off their powers in more than just one fight, and if they had survived, these villains could become longtime rivals of the hero. Which anime characters died just as things were getting good?
10 Lust, The Homunculus Assassin (Fullmetal Alchemist: Brotherhood)
The archvillain, Father, created seven homunculi "children," which included the assassin known as Lust. This deadly beauty could use her charms and good looks to blend into high society, get information out of people, or simply kill them. She also has a cool combat ability and solid leadership skills.
Lust did really stole the show early in this series, but after she tangled with Colonel Roy Mustang, her time was up. Lust got burned to cinders, and fans would never again see her seduce and manipulate people or use those cool stabby fingers of hers—what a shame.
9 Sasori Of The Red Sand (Naruto)
The villainous Akatsuki organization had ten members, and this included Sasori, the master of puppet jutsu. He even turned his own body into a deadly puppet loaded with weapons. He was once the companion of Deidara, the master of explosions.
Sasori was eerie and cool during his short time in the series, and while his puppet jutsu was incredible, it didn't save him from the Sakura/Chiyo team, and Sasori was wiped out. He made a later appearance, but really, he ought to have had more than one good fight scene. His collection of puppets is huge; why not explore it a bit more?
8 Aaroniero Arurruerie, The 9th Espada (Bleach)
In many ways, Aaroniero Arurruerie the Espada is like Sasori. He had only one good fight scene, and he had a "collection" of his own. Instead of collecting puppets, Aaroniero ate 33,650 other Hollows and added their strength and powers to his own. This included Metastacia, which in turn had once absorbed Kaien Shiba.
In theory, Aaroniero is a one-person army despite being a mere Gillian-class Menos. He could have rotated through many different cool powers in his fight with Rukia Kuchiki and (potentially) other foes. But that never happened. Aaroniero let down his guard, and Rukia fatally stabbed Aaroniero's glass head.
7 Lord Boros, The Alien Commander (One-Punch Man)
In many regards, Loros Boros the alien was the Saitama of his own galaxy. He is a mighty melee warrior who longs for one thing: a truly worthy rival who can challenge him. He has scoured the universe seeking such a foe.
Finally, Boros met his match: Saitama, the caped baldy. Boros fought well, but he died at Saitama's hands, which is a shame. Imagine if he narrowly escaped, then formed a coalition of supervillains to challenge Saitama once again. That would also be a fine opportunity to deepen Boros' character further.
6 Magne, The Magnetic Villain (My Hero Academia)
The League of Villains is made up of many powerful fighters, and one of them was the melee warrior Magne. This brawny villain could fight Tiger of the Wild, Wild Pussycats on even terms, saying a lot. Magne also had a unique magnetic Quirk, drawing men and women together across any distance.
This power can draw in fleeing opponents or yank an ally out of harm's way. The possibilities are many, but fans will never see them because Magne soon died at the hands of Overhaul, the Yakuza boss.
5 Pesci, Wielder Of Beach Boy (JoJo's Bizarre Adventure)
During the Golden Wind story arc, Bruno Bucciarati's team boarded a train, only to run into two clever Stand-wielding assassins: Prosciutto and Pesci. Both of them died at the hands of Bruno's squad.
During this two-episode battle, Pesci went from a meek and goofy fellow to a cold, deadly assassin, and the transformation was remarkable. It's too bad that he died because if Pesci cut his losses and escaped, he could have returned with even greater resolve and used his creative Stand one again. He was too cool to die this soon.
4 Yahaba, The Eye Demon (Demon Slayer)
Early in the story, Tanjiro Kamado faced two mighty demons: Yahaba, and Susamaru, who played off each other well. Yahaba had his eyes on his hands, but not just to see. He can also move objects around with his eyes' arrow-shaped irises.
Yahaba can't compare to the Twelve Demon Moons, but he was still a creative and exciting villain, and after Susamaru's defeat, Yahaba should have fled and teamed up with someone else. That would have been a lot of fun, but instead, he fought Tanjiro and got killed in his only battle scene.
3 Yamori, The 13th Ward's Jason (Tokyo Ghoul)
Yamori was one of several terrifying Aogiri Tree terrorist organization members, and he appeared partway through the story as a cruel torturer and assassin. He captured Ken at Anteiku, then tortured Ken nonstop.
It was a relief when the brutal Yamori was killed at Ken's hands, but then again, what if he survived and resumed operations as "Jason," the killer? A rematch with Ken would have been fantastic to watch, but in the actual story, Ken killed Yamori, then fed off his flesh to gain more power.
2 Raye Penbr, The Hapless FBI Agent (Death Note)
Strictly speaking, Raye Penber was a minor antagonist in this story rather than a true villain. But he did pose a threat to Light Yagami, and sure enough, Light took care of the situation the best way he knew how.
Light convinced Raye to show him his FBI badge, and Light used the Death Note's powers to control Raye and make him kill the other FBI agents in Japan. Then Raye died, a hapless and noble fellow who was only doing his duty. If only he had gotten out of this alive somehow.
1 Beldia, The Annoyed Headless Horseman (Konosuba)
Beldia was one of the Demon King's top minions, a headless horseman with considerable power. He lived in a seemingly abandoned castle, and Megumin kept trying to blow it up, annoying Beldia endlessly.
Eventually, Beldia arrived at Axel Town's gates and demanded to meet his tormentor. Despite his great power, Beldia was defeated, and it's a shame. He was such a cool character; imagine if he became Kazuma's and Megimin's nemesis, comically stalking them to find an opportunity to strike.