For all I know, revenge only does more harm. Hatred will ultimately destroy you from the inside. By reducing another being to less than human status for whatever reason, you are travelling down a very dangerous, a horrible road.
By denying an act of compassion or mercy to a dying man, what good can you achieve? For yourself, for any of his victims, for those he did not kill but made victims nonetheless?
If your concern is for the victims, do something for them. Don't invest your time and energy into hatred or revenge, but into doing good for the victims. For any victim you might know.
So much effort and money is invested into finding the guilty and dealing out what we perceive or define as justice by the law. Money is made, too, if possible, a lot. Books and books are published about the one who commits the crime, films are made. The details of the crime are unravelled, analysed, the parts that might sell are not left alone until they are entirely sucked dry. Everybody seems obsessed with the atrocities commited and the criminal, be it a rapist, child abuser, torturer, murderer, war criminal, terrorist... Where, in all this, is the victim's place? It is pretty easy to cry hatred and revenge against someone who committed a crime. But whom does it do any good?
Once the trial is over and the hype dies down, what becomes of the victims? Or of those indirect victims, relatives and friends, who will live with the crime committed against a loved one or even by a loved one? What is done for them? When it rips their lives and hearts and families apart, who is there to help them? What could they possibly gain from all the waves of hatred washing over the guilty one?
How can an act of compassion as this, a minimum of mercy shown to a dying human, feel so wrong to some? Dehumanising and vengeful hating, coming up with terrible ideas of what suffering should be dealt out against another being - it makes me sad and sick to the bone, no matter against whom it is directed. More, it makes me angry, if it is done in the name of victims.
After a crime was committed, you can never hope to get even. There is no such thing as and act of justice or a punishment that will level things out, make up for the damage. As you cannot undo it, you can never make up for it, you just cannot, ever. But you can help, you can ease suffering, you can be there, support, protect, encourage, make life seem worth living again, help to heal. Is not that a thousand times better? I should think so.