तस्य तात वधादेव देवदानवमानवाः ।मधुसूदन इत्याहुर्ऋषयश्च जनार्दनम् ॥इति महाभारते ॥
In plain words
It was that killing, dear one, and nothing else: because he struck Madhu down, gods, demons and human beings — and the sages along with them — call Janārdana by the name Madhusūdana. So the Mahābhārata says.
Close to the Sanskrit
From the slaying of him, dear one, indeed, gods, demons and humans, and the sages too, call Janārdana 'Madhusūdana'. Thus in the Mahābhārata.