As guilty as I feel to answer this question, there are a few ways to die without pain.
• shooting yourself in the head. A bullet travels so fast that your skull would just expload and you wouldn't feel a thing.
• jumping off a buliding. If you land on your feet though, you might feel like a few seconds of pain under every bone in your body breaks. If you land head first, you don't feel anything due to the impact it has.
• if you hung yourself from a high buildin and let youself fall, no pain would be expreience because the incapability to breath isn't what kills you, it's actuallythe rope snapping your neck. But say if you hubg yourself from a low point, you'd die of suffocation which hurts.
• I dont think you can kill yourself this way, but if you got hit by a nuke, no pain would be experienced because you'd just disintegrate into nothing.
I really hope you dont do anything bad or try to commit suicide, xx