I think Snape really is more of an INTJ because he's both innovative and methodical. Ns, especially Introverted Ns. also tend to be bookish and research oriented, and this is very true of Snape.
INTJs can also be rather loyal -- that's not exclusive to SJs. For example, I tend to do what I'm told by other people because it affords me more private time afterwards, or perhaps, I show loyalty to someone because I see a sort of strategic purpose.
INTJs are also quite snappy because, more than any other type, they tend to get impatient when they keep having to explain things.
Oh, and did I mention that Snape is really sarcastic? That's pretty INTJ-ish, for you.