I first thought in my sleep but then I thought about it. You hear someone say that the person died peacefully in their sleep, how do they know it was peaceful, how do they know that they did not wake up having a heard attack and it was not so peaceful. How about in my sleep when I am so zonked out that I don't wake up when it happens.

nitrogen

something painless probably

Death because of old age.

Death by laughter

By a sword through heart⚔️