The thing is, OP, we can't bring back the death penalty unless we also repeal our own domestic human rights laws and withdraw from all our international human rights obligations. Because the number one human right is the right to life.
Be careful what you wish for.
The whole point of human rights is that all humans are supposed to have them. Not just humans who haven't committed serious crimes. All humans.
As soon as you start saying that not all humans are entitled to have their human rights respected, it's the thin end of the wedge. If murderers and rapists don't deserve human rights today, who might be judged undeserving of them tomorrow? People who have committed lesser crimes? People with the wrong political beliefs?
Come for murderers' human rights today and who knows who will be coming after your human rights tomorrow.
And if we're going to withdraw from all our human rights obligations just so we can execute terrorists and paedophiles, how do the rest of us then ensure that the human rights we take for granted are respected? Such as our right to freedom of expression or belief. Or our right to freedom from torture and degrading treatment. Or our right to privacy and a family life. They're all up for grabs now.
What about our right to a fair trial? Because if you're going to bring back the death penalty it's doubly important that everyone has the right to a fair trial, but by getting rid of all of our human rights obligations in order to bring back the death penalty, you've compromised all of our human rights including the right to a fair trial, making miscarriages of justice more likely.
There's also no evidence that the death penalty serves any useful purpose. It doesn't appear to serve as a deterrent to committing violent crimes. (Look what happens in America.) There's always the possibility of miscarriages of justice. Juries become reluctant to convict criminals even where the evidence is clear because they don't want to be responsible for someone's death, so more criminals actually walk free.
And when you look at the people who are actually executed in countries like America, they're almost always incredibly vulnerable people with horrific histories of abuse, low intelligence and no access to decent lawyers. They're people that have been allowed to slip through the cracks of society, and then they commit atrocities and society wants to extinguish them so they're no longer a problem.