Yeah, I know, the TNG characters needed a bit of a kick in the pants on a good day, and it would have been fun to see her stick around and deliver one to a character who needed it (to me, that was part of the fun of the Worf/Jadzia relationship on DS9).
Still, the story they wanted to tell needed her death to motivate it. While it could have been told a bit better in some aspects, I can't see how she couldn't have been left alive and still give Worf the same desire - when it was just his honor at stake, he didn't have a problem with leaving his family honor tarnished, knowing it was for a good cause, keeping the Empire together. But with K'mpec dead, Duras close to getting his job, AND now a son, who he has to take care of with K'Ehleyr's death, he wants his son to be able to be accepted in Klingon society. If K'Ehleyr had lived, she could give him a life that didn't tie back to the Klingon beliefs, but Worf doesn't think like that - you are a Klingon, you must be raised Klingon. And the Klingon beliefs said that with the House of Mogh dishonored, Alexander too would be dishonored and unable to survive in Klingon society. Worf accepted that, but he wanted Alexander to have the option to join Klingon society.