Technically, the writers didn't want that. It was Michael Dorn's idea. He was talking with one of the writers about what would motivate Worf to kill Duras, and Dorn told him that would only happen if Duras had killed K'Ehleyr. He apologized to Susie Plakson on the set for getting her character killed.
Source? According to Michael Piller, he came up with the idea.