There are many ways of killing people and humanity seems to have a taste for death closely followed by a taste for mourning; this is what we learn from great wars. However, in peacetime, some nations kill their citizens as a result of a judicial process. I have always thought the death penalty as barbaric, uncivilised and flawed. It certainly is flawed in that mistakes happen and when a person is executed it is too late to make restitution if it is found that the person should not have been executed.
The United States of America seems to proudly boast of its propensity to execute people. It is one of the few developed nations which retain and almost gleefully implement the death penalty. Each state in America has different rules about execution. the most common method is by lethal injection, but surprisingly the USA does not manufacture the chemicals required for lethal injections and European nations which do make them are becoming increasingly reluctant to allow these chemicals to be exported to enable the USA to execute people.
As a result of a shortage of chemicals Oklahoma is seeking to re-institute the firing squad as a means of execution. It is also seeking to allow execution by placing a person in a nitrogen chamber, so that the person may suffocate to death, apparently painlessly.
So far humanity has used beheadings, stoning, the garotte, the guillotine, the noose, the firing squad, lethal injections and electrocution as means of ending life for those found to suffer the death sentence by some judicial process. Humans are relatively frail. Killing them is an easy option and one that once done is irretrievable. It is fundamentally unjust.