Imprisonment and execution are, regardless of your opinion of the death penalty, imposed as a result of due legal process - a human right under Articles 7, 8, 9, 10 and 11 of the Universal Declaration of Human Rights, to which the United States is a signatory. If you deny them these Articles, you also deny them Article 5, which prevents torture and degrading or inhuman treatment, Article 2, which guarantees freedom from discrimination, and Article 4, which prevents slavery.
So great! Now that you've declared a person nonhuman, you have your perfect little sentient toy that you can keep, chain, torture and kill at your whim without any regard for those pesky annoyances like conscience.
If you are okay with this then I'd say that you are the one being inhuman. There's a reason the Universal Declaration forms a cornerstone of human rights law all over the world.
ALL human beings. All. No exceptions.