I know standards of morality have changed over the years. I do find it a tad hard to believe that as recently as Edison, it was still morally acceptable to execute dogs, cats, and an elephant to make a minor point for one's financial gain. Yecch!!

As for modern execution. 37 of the 38 states that have the death penalty use lethal injection now. This is far better than electrocution because first they give an injection making the prisoner unable to show any pain then they give the drug that causes death. So, s/he can't show pain and what we don't see must not have hurt him/her ... or something.

Tangent: What I really love is that we have a major party whose platform includes that abortion should only be legal when state mandated after the seventy-fifth trimester. (Do the math; it works.)

If you believe that was the worst thing Edison did, you don't know much about Edison.

http://www.lateralscience.co.uk/edison/index.html

As for executions, you are quite correct, the pain to the executed is secondary to the impression left on the witnesses. Animals injected with potassium chloride solution for classroom dissection give a single jerk and die, but apparently that jerk is too much for our delicate nerves. For that matter, it would be very simple to just give an overdose of heroin, but the (conservative morality driven) law won't allow that because the executed might become addicted.