political messages (spoilers)
frank got a message from god to go 'fight evil'. his methods and his experience totally under-prepared him for what he would need to do to accomplish such a task. as a result, he essentially becomes a mass murderer and violent criminal himself.
on top of this, he completely ignored the social institutions that have been built up over thousands of years that attempt to deal with the duality of mankind and his violent nature. the police, for example, are, in theory, accountable to the taxpayers. there is a branch of government for law enforcement, and it is separated from the branch of government that deals with judicial issues, and that is separate from the branch that makes laws. this system was designed to prevent exactly the types of abuses that Libby and Frank perpetrate against innocent people, including use of excessive force, lack of evidence, inability to even identify a suspect properly, etc etc etc.
if you read the news, you will find analogues of everything Frank and Libby did in the actions of police and military forces across the world - attacking innocent people based on mistaken identity, killing people without reason or need to, beating people near to death for minor infractions, etc etc etc.
there is also the blatant inability of the people in the film, including Libby and the Drug Dealer, to accept responsibility for anything, with specific lines of dialogue arguing their lack of culpability for the damage they inflict. of course, any organization that harms the innocent will tend towards this sort of sociopathic inability to feel sorrow or empathy for its victims. Libby is almost the embodiment in a human being of what happens when you remove ethical considerations from the decision making process.
most of all, consider Libby's death. if this were a war film, you would expect it to happen. if it were a cop film, you might half expect it to happen. somehow because of the tone of the first hour+, you dont expect it to happen quite like it does in this film. but in real life that is exactly how death happens. and that is exactly what war is, every single time it is fought, there are deaths like Libby's. somehow people dont connect the 'shock' of deaths like that to the 'tone' of a war's beginning, just as the 'tone' of this film makes the viewer unready for the obvious consequences of the characters actions.