I found this site linked to the Washington Post.
It looks at how many were executed in the USA but also how many people they murdered. In general there were far more innocent persons murdered by those that were executed. It's not an eye for an eye.
Also bear in mind that prosecutor's seldom prosecute someone for every murder they have committed. One that is air tight is sufficient for a murder conviction.
http://www.washingtonpost.com/wp-srv/special/outlook/death-penalty/?hpid=z4
John Wayne Gacy was executed for 12 of the 33 boys he murdered. The DC sniper(s) John Muhammad was executed for 1 person although he and his accomplice executed 12 and were linked to more murders around the country. Muhammad's companion confessed to 7 additional murders and in exchange got a life sentence.
As I look over these numbers I was struck by the thought that a lot of the executed had murdered more then one person. In Timothy McVeigh's case he killed 168 persons....19 of which were children in the Oklahoma Bombing.
Like it or not I don't see any sense in keeping a murderer alive if he has not been co-operative in recovery of buried bodies and accomplices.
As far as the manner of their death goes we need to stop over thinking it. Hangings can go wrong. The Electric Chair was a failure at times, the gas chamber looked medieval, now these death drugs are malfunctioning.
The most merciful would have to be the firing squad. 6 or 8 will fire only one will have a live round.....but will not know it. Otherwise if the evidence is spot on DNA and confessions etc........I don't see a life term. The Richard Speck's case comes to mind The killer of 8 Nurses one horrific night received a life sentence. A sex tape was made of him engaging with prison friends...... in the tape Speck says "If they knew how much fun I was having in here, they'd have to turn me loose."
There are many other mas murders who received life sentences an now have their own fan clubs. A life sentance is not the ultimate punishment its a stay of execution with benefits.