The first thing there is to say about this character is that typing him is a tad problematic: Just because he's a serial killer does not mean I, in any way, intend to perpetuate stereotypes. I'm not saying Ts are always the cold-hearted killers or that Fs can't be; I'm saying...Johnny is INTP. Besides, for the first half of the series, he's under the control of some underworld kingpin whose job it is to possess one human being who will dispose of as many people as possible who are a burden to human civilization.
I will start by looking at the things he hates. The same way INFPs can often focus their Fi values on criticisms of outward logical structures and systems (Te), (e.g., the not-so-humane way a company is run, police brutality) INTPs can focus their Ti principles on criticism directed at outward social structures (Fe). Johnny is an extreme case of a hater of society -- even of human civilization, making him a misanthrope. He finds countless flaws in his society, finding people guilty of irrational, animalistic behavior. He longs for a world in which Homo sapien would live up to its name.
Because he values rationality, logic, and knowledge, above all, and because he's so clearly an Introvert residing namely within the confines of his own mind, he's thus far IT. When we add in his paradoxical hatred of, and unrelenting focus on, human society, he becomes ITP. He also seeks to eliminate all emotion from his being, aspiring to become "as cold as the moon." All this is indicative of an ITP who has yet to begin to integrate Inferior Fe, and is still caught in the throes of that love/hate relationship.
Indeed, Johnny is a familiar embodiment of that archetype, The Thinker, concerned with finding out the truth of his reality, understanding that every "solution" is merely another name for "more problems." Ti is a right-brain function, and, as such, is wholistic in its understanding, not wanting to leave any nook or corner unlit, unable to distinguish between a whole and its parts. As such, he is ever subject to, as the INTP archetype goes, excruciating self-doubt that arises from a keen awareness of the inevitable gray areas in personal understanding. Just as Fi understands, and can pick apart, more than systematized Fe, subtle feeling tones and nuance, so does Ti more often pick up on minute inconsistency and dwarf contradictions in its workings than expedient and efficient Te.
Johnny's killing sprees in the first half of the series might indicate some kind of out-of-control Se, but Johnny wasn't quite himself in this half because he was under mind-control. When he comes back to life as his real self, he's much more in control and sedentary, spending most of his time pondering how to make his life better.
From my readings, I've gathered that, while the greatest antipathy is most often felt for the Inferior, a similar sentiment can exist for the Tertiary if the person has not gone through the integration process. I've mentioned that Johnny shuns and wishes to fully shut out all emotion from his functioning, but to a slightly lesser extent he also seeks to shut out his physical needs. He hates sleep, hates the need for food, the need for sex...pretty much the need for anything. If he could, he would become fully emancipated from his body.
Because he harbors disdain for what equates to his Feeling function (Fe) and his Sensing Function (Si), the two he operates on most of the time must be Thinking and Intuition -- in that order. Indeed, Johnny's thoughts, when expressed, are very abstract and deal with big picture topics.
Johnny is an INTP who is under mind-control for the first half of his series, which makes him somewhat unrecognizable for that term. Once he is reborn, however, he is much more lucid and verily resembles an INTP working on becoming a more healthy, balanced individual.