My mother spanked me when I misbehaved. She caused real pain as punishment.
It was never cruel.
Punishment and cruelty are not equivalent.
Please note, I am not in favor of spanking children, and never spanked mine.
Now as to your final point, that justice is the state taking revenge on behalf of the victims. I replied to you above that I see the validity of this, and for the same reasons you give (discourage eternal feuds). And for those same reasons Justice is restraint. At most equivalent harm, no more than an eye for an eye.
I think it was your anger that made you say these ethics we both share are bullshit rules designed to hurt your family.
Now in the current case, for that point to stand, we need to ensure that the prisoners have in fact harmed identifiable others. Google tells me "Roughly 40% of inmates in Texas prisons are incarcerated for non-violent offenses". Thus 40% of the time, there is no cause for vengeance.
And where prison is vengeance, if it is just, it needs to be restrained. Not cruel. Not unusual. Just as punishment is not cruelty, justice is not cruelty.
For other readers, as I mentioned above, while I see a valid ethical stance behind NoMoreNicksLeft's anger and hurt, I do not share it. I seek mercy, and as for Justice, "Justice is mine sayeth the Lord"-- in other words, I leave it to God. I find it sick that the US, the "land of the free" has the highest incarceration rate in the world. Look, I've had bad interactions with bonafide unredeemable unrehabilitable psychopaths, and I'd like to see such people locked up for life. But I do not wish them cruelty beyond the innate cruelty of life behind bars.