He knows Irvin's habits, the food he likes, his favorite brothel, his favorite alcoholic beverage, and many people with known connections to him. So he has a direction in which he should go looking for him.
He took the picture of the Scarred Raven that was drawn on paper with a black pencil and went about it with it in search of his targets.
He would beat people and torture them until they gave him information on who he was looking for. Then he would kill them.
The Scarred Raven's associates are criminals. They are either parts of gangs or doing even worse business that they didn't have the guts to make public by forming a gang.
So he didn't feel guilty about killing them. In fact, he was happy to do it because these people tended to have money too.