The saddest flight I've ever made was some years ago in Bayern. Perfect conditions for some mountain flying.
I was on frequency when an exchange between a solo pilot and ground station made it clear that the pilot had no intention of deviating from a collision course with a mountain.
I could not see the aircraft, but saw the resulting fireball as he flew full pelt into a sheer rock face.
I still wonder if I could have made a difference by speaking to that pilot.