What if you let this person go without knowledge of what he did before, and he got out and did it to 20 more people.
What if he was a child molester? Kidnapped children, tortured for weeks locked in his basement, raped and killed?
How would you feel then?
What if you were confronted with the parents, aunts, uncles, grandparents of the children brutally murdered? And let's assume they have the knowledge of the fact that you are the person that set them free?
Obviously any sane person would feel bad if this person committed a string of additional crimes. My mother (who is a judge) once released a man on bail for spousal abuse, and he promptly went home and murdered his wife. Obviously my mother was devastated, but that did not make her decision as to bail the wrong one.
That said, I would feel a whole lot worse if I killed the convict and he was later exonerated by DNA evidence. IMO you cannot justify any important decision by its results if it is wrong at the time it is made.