Since Snape is a fictional character, what we know about him is strongly colored by the perspective of the books in which he appears. Many discussions of Snape, on forums, fansites, and elsewhere, have hinged on the fact that we see him primarily through Harry's eyes, in situations that Harry will be aware of. Wondering what he is doing the rest of the time is not rationalizing, but speculation. I have seen arguments that he is ISTJ before, some better stated than others. I do not "need" to see Snape as an INTJ, I simply do, perhaps because I am one, and much about him resonates with me in a way that more definitively ISTJ characters (and real people) do not. I summarized a few of my reasons, as those who see him as other types have done.