Killing the body is problably one of the easiest things to do.
Jump in front of a train. Jump off of the nearest tall hotel. Making a run for it should be enough. Put a sturdy rope around your neck, tie it to a tree and jump. Drink a gallon of bleach. Set yourself of fire holding a gasoline tank. Set a powerful grenade off in your mouth. C4 all around your head and press the button. Make an automatic machine that slices your neck with a katana(sounds decent actually)
Anyways, my point is. There are more things to it than just killing the body. Most people don't want to trsumatize those left behind too much. They want to leave a decent looking body behind. They also want it to be painless and comfortable. At least as much as possible. They also want to be ready for it mentally....etc
So much to it
I wouldn't say "killing the body" is something that's easy, especially not probably one of the easiest things to do.
Not only will the survival instinct kick in in about 90% of the methods you described, some of them are literally impossible to achieve. Like, mate, how can you get C4 that easily? How can you even get a katana, and how the fuck do you make a decapitation machine with it? Things like explosives and swords aren't that easy to obtain, you know.
And the bleach, fire, jumping, hanging and train methods? Fire is agonizing. Bleach isn't effective and also hurts a whole fucking lot. Most don't have the courage to jump in front of a train. That thing moves pretty damn fast and is guaranteed to leave a bloody mess. Jumping has a pretty high chance of failure. Getting hanging right is difficult and if done wrong, you can end up with some serious injuries.
My point is: No, it's not easy.