Talking about a 'fair trial' only makes sense when dealing with issues in the realm of criminal law.
But drone strikes and other military actions are distinctly outside the realm of criminal law. It is a category error to try to apply the concepts of a 'fair trial' to these situations.
There are entirely different set of rules and agreements that are associated with armed conflict. Even the moral/ethical arguments are entirely different within the context of armed conflict vs criminal law.
One of the reasons our public discussion on these issues is so muddled is that these two contexts are often confused.
As an example, many critics of the US policy regarding detainees at Guantanamo Bay will argue that we have failed to give them a 'fair trial'. But that is attempting to insert criminal law concepts into an armed conflict. It isn't necessary to prove that a crime has been committed in order to detain people in an armed conflict. Different rules apply.
I'm not suggesting that there aren't arguments to be made against the detainee program, but those arguments need to be made in the scope of the legal framework accorded armed conflict not in the scope of the legal framework for criminal activities.