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.
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)
Kenny Ackerman (Attack on Titan)
Levi Ackerman (Attack on Titan)
Amon (Amon The Apocalypse of Devilman)
Hansel & Gretel (Black Lagoon)
Roberta (Black Lagoon)
Akira (Devilman Crybaby)
Goblin Slayer (Goblin Slayer)
DIO Brando (JoJo's Bizarre Adventure)
Pannacotta Fugo (JoJo's Bizarre Adventure Part V: Golden Wind/Vento Aureo)
Kan Ki (Kingdom)
Kyou Kai (Kingdom)
Mountain Clans (Kingdom)
Kaido (One Piece)
Charlotte Linlin/Big Mom (One Piece)
Jack (One Piece)
Sanji (One Piece)
Mikoshiba Ryouma (Record of Wortenia War)
Kunugi Renya (Second Life in Another World)
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.
When he was possessed by Amon, Akira (Devilman Crybaby) inherited both the strength and brutality of a devil.
Grayson Hunt (Bulletstorm)
Doom Slayer (DOOM) has no problem slaying entire armies of demons through sheer brutality, even punching through them with his bare hands.
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.
With sheer brutality and determination, Kratos (God of War) brought about the downfall of Olympus and mercilessly killed the greek pantheon.
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.