Users are incredibly skilled at harming others, whether intentionally or unintentional. Users have no problem using any means to get their way with others, even if it involves breaking a few bones or leaving some scars. They don't necessarily have to be good at fighting or killing to be violent, however, the intention to harm has to be there, which usually does lead to the deaths or maiming of others.
Pannacotta Fugo (JoJo's Bizarre Adventure Part V/5: Golden Wind/Vento Aureo)
Mou Bu (Kingdom)
Kan Ki (Kingdom)
Kyou Kai (Kingdom)
Mountain Clans (Kingdom)
Zen Ou (Kingdom)
Various Characters (Kengan Ashura)
Gabimaru (Hell’s Paradise: Jigokuraku)
Aza Chōbē (Hell’s Paradise: Jigokuraku)
Kaido (One Piece)
Charlotte Linlin/Big Mom (One Piece)
Jack (One Piece)
Sanji (One Piece)
Mikoshiba Ryouma (Record of Wortenia War)
Gintoki Sakata (Gintama)
Yato Clan (Gintama)
Sugimoto Saichi (Golden Kamuy)
The Boys (The Boys)
The Female (of the Species)
The Joker (DC Comics)
Teth-Adam/Black Adam (DC Comics)
Slade Wilson/Deathstroke (DC Comics)
Jonah Hex (DC Comics)
Lobo (DC Comics)
Constantine Drakon (DC Comics)
Various Characters (Sin City)
Albert "Al" Simmons/Spawn (Image Comics)\
Kurr/Savage Dragon (Image Comics); before losing his memory.
Conquest (Image Comics)
Barracuda (Marvel: MAX Comics)
Cletus Kasady/Carnage (Marvel Comics)
Wade Wilson/Deadpool (Marvel Comics)
M-11/Human Robot (Marvel Comics)
Frank Castle/Punisher (Marvel Comics)
Victor Creed/Sabretooth (Marvel Comics)
James "Logan" Howlett/Wolverine (Marvel Comics)
Practitioners of the Hercules Method (The Strange Talent of Luther Strode)
Brock Samson (The Venture Bros.)
Edward Blake/The Comedian (Watchmen)
Walter Kovacs/Rorschach (Watchmen)
Midnighter (WildStorm/DC Comics)
Sheriff Lucas Hood (Banshee)
Jason Voorhees (Friday the 13th)
Eric Northman (True Blood)
Brandon Breyer/Brightburn (Brightburn)
The Penitent One (Blasphemous)
Grayson Hunt (Bulletstorm)
Dante Alighieri (Dante's Inferno)
Isaac Clarke (Dead Space)
Necromorphs (Dead Space)
Doom Slayer (DOOM)
Kratos (God of War)
Various Characters (Hotline Miami)
Jack Cayman (MadWorld)
Various Characters (Mortal Kombat)
Ryu Hayabusa (Ninja Gaiden)
Shinobu Jacob (No More Heroes)
Travis Touchdown (No More Heroes)
Alex Mercer (Prototype)
James Heller (Prototype 2)
Rick Taylor (Splatterhouse)
William J Blazkowicz (Wolfenstein)
Viltrumites (Image Comics)
Amon (Amon The Apocalypse of Devilman) is considered by most if not all demons in existence as the most powerful and the cruelest devil in the Devilman Universe.
In his younger days, Kenny Ackerman (Attack On Titan) was a ruthless serial killer in the capital of Wall Mitras as he slit the throats of over 100 members of the Military Police Brigade as they hunted him and his family.
When he was possessed by Amon, Akira (Devilman Crybaby) inherited both the strength and brutality of a devil.
Goblin Slayer (Goblin Slayer) is well-known for killing goblins in extremely brutal ways, including burning and drowning them out. He has even made goblins soil themselves from him threatening their lives.
Unreasonably short-tempered and always ready to shed blood, Jack the Drought (One Piece) has no qualms in causing unnecessary destruction while asking simple questions, even when there's no real benefit to it.
DIO Brando (JoJo's Bizarre Adventure)
During his time in his village, Gabimaru (Hell’s Paradise: Jigokuraku) was well known for his cruelty and the ruthless ways he would kill his victims.
As a Bushin, Hou Ken (Kingdom) was fully capable of demonstrating extreme levels of violence while following his path...
With strength that's been called monstrous, Mou Bu (Kingdom) is one of the brutal warriors of the State of Qin...
With rage that's hot enough to melt even rock, Kan Ki, the Beheader (Kingdom) is utterly brutal towards anyone or anything...
...and as a general of Qin, he can unleash unparalleled levels of violence while achieving a total victory on the battlefield.