No i dont mean gameplay, im talking about which ones are the most irredeemable ones (to avoid bias and the obvious "one", we arent including Licensed characters, BESIDES Ghostface due to being a special case)
In my opinion Knight, Clown and Doctor easily make it to the top 3.
Knight is essentially a psychopathic warmonger, driven by pure violence that refuses any notion of peace or laws, brutally killing anyone that stands in his way just for the sake of it, this is made even more apparent when he outright claims that if a "peaceful paradise" that Vittorio was looking for did existed that he wanted nothing to do with it.
Clown is a sadistic murderer that even had a chance to redeem himself and change his ways for a better life, which he refused cause it was "Boring", he also kills innocent people just cause they remind him of people that "wronged" him in the past.
Doctor loves to torture other humans for the sake of "knowledge", and makes no attempts to hide his sadism, so much so that prisoners begged to not be sent to him specifically, he grew increasingly worse the more he experimented, even turning on his mentor who became one of his many victims.
Interestingly enough, none of the 3 seem to have any real reason for being this way, Knight was born an unfeeling monster that didnt even seem to cared for the people in his village being massacred, Clown did had a neglectful father, but its made very apparent that he had serious problems since a being a kid (And again, he could have walked a better path that he refused out of boredom), and Doctor is always portrayed has someone that doesnt care for morals and is inherently sadistic.
So yeah, these 3 are my top pics has the most irredeemable killers in DBD lore.