John Rogers: Understanding the Antagonist

Some antagonists see the world with a view that differs
 in only one or two areas; but then there are some
who see everything in reverse--and those are the
tough ones to write!

"You don't really understand the antagonist until you understand why he's a protagonist in his own version of the world."

~John Rogers

    Writing antagonists can be tough. Because we want to write characters that are realistic and relatable on at least a few matters, but it's difficult to relate to someone who does horrible things and has a poisoned perspective on life. And when someone's mindset is that foreign to you, writing about them is like describing a place you've never been. The risk of using clichés and cheesy dialogue is probably at least twice as large for antagonists than it is for other characters. So it's pretty helpful how John Rogers phrases the claim of his quote so simply; that the key to understanding an antagonist is to understand why they think they're right.
    There are basically two ways to understand something: intellectually, or personally. Between them, understanding intellectually seems a lot easier. Because it's just gathering data from observations and maybe some backstory and putting it together to form a conclusion, a simple label. President Snow from Suzanne Collins's Hunger Games, for example, sends children to their deaths to remind the Districts of the Capitol's strength, and uses the surviving "tributes" as pawns for his own gain. Diagnosis: he is power-hungry, and his heart is too hardened to show any sympathy. Coming up with that conclusion was easy; it just involved looking at his actions and making connections.
    But how would you come to understand why he's a hero in his own mind? How would you understand his actual thoughts on his behavior, how he justifies his actions so he can live with himself? This would take some digging that goes beyond the surface of his actions; it would call for actually knowing what he's thinking personally, being aware of how he treats matters when he's not putting on a show for others. One way that might shed light on such personal thoughts would be to write a scene where the antagonist is acting outside of the story, out of view of the other characters, just doing what s/he does in regular life. Another way to understand their beliefs is to hear it from their own lips--maybe in an interview-style dialogue. Those bring out the distinctness of their personality, and they're good for learning new things about the character.
    So, have you ever given much thought to understanding the mindset of an antagonist on a personal level? Do you take extra time before starting the story to write something where they explain why they think they're in the right?