Where, in the Constitution, does it say that our government should gun down anyone, armed or unarmed?

Armed or unarmed, guilty or not, charged with a crime based on the preponderance of evidence or not, what gives our government the right to murder anyone, outside the United States? Regardless of how many terrorists acts he's committed, we have the rule of law here. So, why do Americans put up with this kind of behavior? It this unconstitutional?