Why is torture unconstitutional?

Answer 1

Torture violates the eighth and the fifth amendments of the U.S constitution.

The Eight Amendment prohibits "cruel and unusual punishment":

Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted.

The Fifth Amendment provides that no person shall be compelled to testify against himself, nor deprived of life, liberty, or property, without due process of law:

No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a grand jury, except in cases arising in the land or naval forces, or in the militia, when in actual service in time of war or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.