Kurn was suicidal. He wanted Worf to kill him; failing that, plan B was to kill himself by any means necessary.
Last time I checked, assisted suicide is illegal even when the victim suffers from an incurable, painful disease.
This is NOT a morally 'clean' thing to do. Quite the opposite.
Now - finding a way for your brother to live instead of snuffing him with a dagger is morally reprehensible?
As for memories - do you remember what you had for lunch on 14 august, when you were 7? We lose memories all the time.
There are examples of people who lost all their memories at once - amnesia. They didn't lose their personality - what made them who they were. They didn't became blank slates, vegetables.
Losing one's memories != dying. NOT even close.