Being hooked and played is stressful to the fish, and some fish WILL die even after the fastest and gentlest release. Just like some people get fatal heart attacks or brain aneurisms when exposed to stress.
Whoever believes that their release techniques are bullet-proof should get their heads out of the dark place they keep it.
All we can do is stress the fish as little as possible and hope for the best. Landing it quickly is the most important part of the equation, so all C&R fishers should have adequate gear to do the job FAST, and not brag how they landed, for example, a 15-pound northern coho on 10 pound test main line and 6 pound tippet). Lastly, and I can't emphasize this enough: fishing IS a blood sport; if you don't want to cause death, find another hobby.