Probably was told about it by either his master/adoptive parent or by himself.
Dont think thats the first time he had a fit but probably when he enters that state he wont stop until everyone near is dead and the bar owner is not keen on that idea so he chooses to finish him off while teaching her a "lesson" at the same time
the backstory chapters for the killers are more interesting than the rest of the plot tbh. i only hope the dumbass protag begins to learn something... eventually??? like the story can't keep saving itself through interesting side characters lol