For, right now people are still not mature enough to forgive and
forget... even if you do let the criminals live, the people around
them have already given them the death sentence.
Moroever, a person given that sentence will be hail as King/Queen
in the prison so the chance of him/her getting the just punishment
is zero.
And like other forumites have said, this will deter others from
committing the crime.