Slaying these sons of Dhritarâshtra, what pleasure can be ours, O Janârdana? Killing these desperadoes, sin will but take hold of us.
In plain words: Arjuna asks how they could ever be happy after killing Dhritarashtra's sons. Even though those cousins have done wrong, he says killing them would only bring sin upon himself and his brothers. Janardana is a name for Krishna.