Mock Trial

I am in an International Law class.  We were asked to hold a mock trial.  We were assigned roles, I was one of the three attorneys defending America.  We were to defend our actions in killing Anwar al-Awalki whom at the time was located in Yemen. I really enjoy the legal field, and would one day like to go to law school so this trial was an assignment I enjoyed.  The question that was asked was, was it legal for the United States to cross another's air space and deploy a missal via drone and kill a US citizen whom has forcefully fought against the US? In our trial the US won in defending its actions, however in real life what is the answer? I personally, not speaking about the legality of it have mixed feelings as most I would assume.  I think that one could fight on either side of the table and make a valid argument.  I would like to think that the US acted appropriately in keeping our citizens safe from extremists looking to physically harm citizens.  But do we feel justified in this action just because it was the United States against another country? What if it had been two completely different countries, would we feel the same?