If a person can't be trusted, do you really need to know the subtle interplay of childhood and life experiences that made them untrustworthy? Practically speaking, no. The stencil is enough to make actionable decisions. As an INFP though, that deeper stuff is what fascinates me, and I can't help but to be drawn to it.
ISTJs have freaked me out on several occasions with how quickly they can sum up the character of a person, good or bad. It usually takes me longer to come up with the same conclusion. I suspect it's the much more objective primary Si/Te.
Would you say that people with EITHER an exceptional or nefarious character generally just sort of 'stick' out to you, almost as if they are a piece that doesn't properly fit in a puzzle?