There are no humane forms of executing someone. Death is painful unless you die in your sleep. The "most humane" forms of execution would be drugs or a bullet to the head, both of which should be administered while the victim is asleep. Executions aren't humane ways to treat humans, but if it is necessary it should be as painless as possible.
Of course, when it comes to hardened killers we may not be looking for the most humane form of execution. But if we rid ourselves of anger and just do what is necessary, we would be only using some form of drug injection that would put the person to sleep. In fact, for many trapped in confused and violent brains, that might be a blessing.