What is Satan's psyche like? Is he a total lunatic?

It depends on what you mean by lunatic. We all have a little craziness in us, some part of our psyche that is a bit off, and it comes out in different ways; and because we are all different, one person's peculiarity can look crazy to someone who lacks it, which is part of why 'normal' is hard to define. This is also where psychology reaches its limits, right at the point where something spiritual is involved that you cannot locate on a chart, the way the old solution was simply a frontal lobotomy that turned the person into a vegetable, since the soul works through the body and a damaged body cannot express it. There is a real relationship between evil and the psyche: spirits can possess people, and some who are possessed appear completely normal and function in society ninety-five percent of the time, until the demon lets loose and they do something truly weird, as exorcists like Father Ripperger will tell you. But the devil is different. He has no body dragging him down; he is a pure spirit, thoroughly and totally evil, with not an ounce of the grace of God in him. Everything he thinks, does, and wills is evil; he is the total antithesis of God. So we would not call him a lunatic. A crazy person does irrational things that may be good, bad, or indifferent, but the devil is not crazy; he is pure, raw evil, and that is something you do not want to mess with.