Scripture DOES provide support for capital punishment, both Old and New Testaments.
It is pretty much based on Scripture that the Catholic Church has historically upheld the morality of capital punishment as a form of justifiable homicide. In other words, it’s not an intrinsic evil.
In fact, the historic catechisms have stated that capital punishment justly administered is not in violation against the command to kill, but is rather “in paramount obedience” to that command, by redressing the wrong.
You are also correct in that it is precisely because of the dignity of both the victim and the offender that the offender can give his life in redress for the offense committed.
The morality of capital punishment, even if one is to hold that on prudential reasons it should not be applied, is part of the Church’s Ordinary Magisterium, and as such, falls under an infallible teaching. It cannot be changed. Even if this Pope or some other will make the attempt, he will be prevented from doing so.
To affirm the inherent morality of capital punishment is not to say that one cannot state that applying it today is imprudent. It is contrary to faith to call capital punishment inherently evil, like abortion and euthanasia. It is perfectly fine to say applying it in this day and age is a really bad idea.
Catechism of the Council of Trent:
"Another kind of lawful slaying belongs to the civil authorities, to whom is entrusted power of life and death, by the legal and judicious exercise of which they punish the guilty and protect the innocent. The just use of this power, far from involving the crime of murder, is an act of paramount obedience to this Commandment which prohibits murder. The end of the Commandment is the preservation and security of human life. Now the punishments inflicted by the civil authority, which is the legitimate avenger of crime, naturally tend to this end, since they give security to life by repressing outrage and violence. "