Why death penalty is not good in civilized society?
Actually, opinions on this vary. But the tendency, nowadays, is
that a majority of people think that the death penalty is
uncivilized. Some arguments against the death penalty is that it is
used disproportionately against ethnic minorities (for example,
black people in the United States), and that it is irreversible
(and there have been many cases where people executed, or people
who were planned to be executed, turned out to be innocent). For
further arguments, both for and against the death penalty, I
suggest you consult the Wikipedia article on "Death penalty".