Morals are subjective, that is correct. There is no universal right or wrong. Instead, good and evil are standards decided by each society for the purposes of defining it. For example, freedom is seen as the ultimate good within a leftist society, while discipline is seen as the ultimate good in a fascist society; neither are better than the other. However, I personally choose to endorse the former because it is more in line with my standards and my morality; however, my standards and my morality are far from universally applied, if you get me.
That being set, thought, a society has to follow the moral standards it sets for itself. Murder is perceived as wrong in the U.S: this is a defining standard. Society, as a whole, must pursue its own moral standards and keep them in all places for the purposes of its definition as a society. Killing is viewed as a very negative act due to our cultural standards; it applies to every single member of the society. It should be avoided whenever possible, and thus jail time is always preferential. The purpose of a jail is to keep the felons who will potentially harm society away; there is no difference between that and killing them, looking from an external factor. The death penalty serves no practical point, you understand, and eliminates the possibility of redemption and rehabilitation.
So you can say I am neither a deontologist nor a consequentialist. I am neither an absolutist nor a relativist. It all specifically depends on the circumstance and the society.
Oh, and before somebody checks my other arguments and deems me a hypocrite for believing that people who break the law (and thus the cultural standards of a society) can only be judged as immoral based on their motivations and the ultimate consequences of their actions? Well, these kinds of people aren't in authority positions- they aren't sworn to, at all costs, monitor the standards of their culture. The judges, the presidents, all of these guys? They're responsible for leading their culture; and a culture is strictly defined by the standards it sets for itself. You bypass these standards, and you destroy a society's moral worth and moral high ground.
Also, something else just occured to me. Going by your standards of morality being purely relativistic and not universal (which I agree with), then the motivations and desires from terrorist organizations and the U.S is inherently equal. Thus, the only way to genuinely create a difference and maintain moral decency for the U.S is to control the means it utilizes. Create strict standards which define our moral decency and our moral policy, because we aren't the "Good Guys".