The current methods of execution are not inhumane. Natural death routinely causes more suffering than executions. What's inhumane is the sentence of death, knowing you are going to die at a predetermined time, the agony of waiting while being incarcerated.
The only humane way for the state to kill somebody is not to tell them they've been sentenced to death. Then, when they least expect it--when they're sleeping, eating a meal, or sitting on the toilet--sneak up on them and shoot 'em.