Actually, there are almost infinite ways to kill a human being effectively, with “effective” defined as making the guy be All Dead.
Painless is pretty easy too. As some have suggested, why not a large overdose of morphine or heroin? Let em go out not only painlessly, but blissfully.
Or use 100% nitrogen. Displaces oxygen, unconciousness is instant, death following shortly. Leave the mask in place for an hour, certify death, bury him.
I vote for this one.