Unless it wasn't Duras himself, but some other random Klingon, that killed her. She only said "Duras" because she figured who was behind it all.
Duras had his men on the Enterprise distract the Starfleet guards so he could leave their quarters. He went to Kheylar's quarters where she confronted him about Worf's discomendation. Next scene Worf walks into the same room and finds her dead... she says his name. Come on now... unless Duras has a twin brother, it was him. Especially when he all but admits it by acknowledging Worf's right for vengeance.