I think the U.S. is a little creepy with all the methods it allows. Hanging and firing squad, yes, but also electric chair, gas chamber, and predominately lethal injection nowadays. The simplest way would be a single bullet to the head, as in:
https://www.youtube.com/watch?v=yvOJgzb52-Y
https://www.youtube.com/watch?v=yvOJgzb52-Y
"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring." - Carl Sagan