I have never been comfortable with the idea of lethal injection, whose purpose seems to be to maintain the decorum of execution, not be aesthetically unpleasing to witnesses, and to look “sanitary” and “painless”.
Practically speaking, most forms of execution, to include hanging, the gas chamber, and the electric chair, all have the potential to go wrong. Only one way, the firing squad, is simple, inexpensive, very reliable, and is unknown in modern practice to fail or injure without killing.
Oddly enough, execution by firing squad is legal in Oklahoma, but only if lethal injection and electrocution have been declared unconstitutional.
Put them in a small chamber and release Freon Gas into the chamber. It is actually a powerful anesthetic and will put you to sleep and as the concentration builds up, the heart will go into a fatal arrhythmia. Game over.