How can one die without any kind of pain?

Pain comes from external sources to your brain, so destroying the brain first prevents it. It has to be quick though, and I think fear is also pretty nasty. You wouldn't want to see it coming, like if you held a gun to your head.
Need a piano to fall on you walking down the street or something.

Wish I knew. It seems like all of us die in pain even people who die of old age.

depression

Everyones death causes pain to someone. Is that your meaning?

You can't

Asphyxia

What's the difference if you feel pain? Dying will end it. What matters more........dying or 2 seconds of pain?

Plastic bag over your head