There isn't a humane way
The concept of humanely killing someone is an odd one. To simply put it, there is no humane way to kill any living being. Death is a messy thing and no matter how we try to disguise it, there simply isn’t a nice way to do it. The current method use of lethal injection is just a theatrical play designed to make us feel better about the whole thing, but some would argue that it is more torturous and painful than other methods such as a firing squad. We must think about what we are trying to accomplish with capitol punishment, are we trying to make the killing humane for the person involved or for those watching? Lethal injection seems to be designed to fit the latter role, but as it has been reported that there have been many botched executions via lethal injection. If then we are focused on the humane killing of the person sitting in death row, then what is the most humane way to do it? There really isn’t one and we should ask ourselves if we should even continue to do it. Ultimately those in favor of capitol punishment would be in favor of the quickest and cleanest method available, despite of what the person being executed will have to go through.