Doesn't he have the right to a trial?
No he has a right to due process, totally different.
I'm just gonna repost what I said in a similar thread a while back, it details the administrations process for targeting suspected terrorists with drone attacks as well as US citizens:
(DOJ White Paper on Drone Strikes/Targeted Killing)
...Lethal force will be used only to prevent or stop attacks against U.S. persons, and even then, only when capture is not feasible and no other reasonable alternatives exist to address the threat effectively. In particular, lethal force will be used outside areas of active hostilities only when the following preconditions are met:
First, there must be a legal basis for using lethal force, whether it is against a senior operational leader of a terrorist organization or the forces that organization is using or intends to use to conduct terrorist attacks.
Second, the U.S. will use lethal force only against a target that poses a continuing, imminent threat to U.S. persons. It is simply not the case that all terrorists pose a continuing, imminent threat to U.S. persons; if a terrorist does not pose such a threat, the U.S. will not use lethal force.
Third, the following criteria must be met before lethal action may be taken:
1) Near certainty that the terrorist target is present;
2) Near certainty that non-combatants will not be injured or killed. (NC's are individuals who may not be made the object of attack under applicable international law. The term "non-combatant" does not include an individual who is part of a belligerent party to an armed conflict, an individual who is taking a direct part in hostilities, or an individual who is targetable in the exercise of national defense. Males of military age may be non-combatants; it is not the case that all military aged males in the vicinity of a target are deemed to be combatants.)
3) An assessment that capture is not feasible at the time of the operation;
4) An assessment that the relevant governmental authorities in the country where action is contemplated cannot or will not effectively address the threat to U.S. persons; and
5) An assessment that no other reasonable alternatives exists to effectively address the threat to U.S. persons.
Finally. whenever the U.S. uses force in foreign territories, international legal principles, including respect for sovereignty and the law of armed conflict, impose important constraints on the ability of the U.S. to act unilaterally and on the way in which the U.S. can use force. The U.S. respects national sovereignty and international law.