I often wonder what is causing us to have such a hard time killing people without pain or distress or the perception of cruelty (as much as that's possible when

I often wonder what is causing us to have such a hard time killing people without pain or distress or the perception of cruelty (as much as that's possible when killing a person). Why aren't prisoners put to sleep with gas and then killed as cheaply (and non-gruesomely) as possible thereafter?