The Fifth Amendment:
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
Note that it says person, not citizen. Your constitution clearly specifies when it applies to citizens only vs when it applies to everyone. If doing it to Americans is wrong, then so is doing it to non-Americans.
Since there is no declaration of war, the possible exception is the "public danger" clause. If it applies, then it applies to everyone.
It could easily be argued that blowing up a US citizen terrorist is less a violation of the constitution than blowing up some non-threatening goat herder on the other side of the world.
If you really want to see disgusting behavior that should get your politicians charged with murder, look up the "double-tap" strategy.