I'd rather not get into specifics but you hit the nail on the head with that bolded sentence. When you're "out", you cease to exist for the IXFP. The judgment has been made. It can be a bit shocking because others don't realize that side is there.
That is more of an ISFP thing though, imo. Ne seems to force me to leave the door open, even a crack.
As for the evil ... certainly I have this awareness of light and dark, the juxtaposition of such things, the integration of such things. Even my avatars reflect that theme with regularity. But I find myself kind of pinioned on your thread title, "IXFP's are surprisingly capable of tremendous acts of evil." First, by the word 'surprisingly'; second the phrase 'tremendous acts of evil'. I find myself not sure how to comment on this.
Are IXFP's human? Yes. Therefore, capable of being either as 'good' or 'bad', light or dark as anyone else. Naturally, there's a mix of positive and negative qualities in us too, just like everybody. What I do think is true of IXFP's is this awareness (even fear actually) that humans are seldom one or the other, exclusively. There's always this mix of grey ... these issues do not compose a black and white world to us. And we are painfully, acutely aware of this mix - we feel inside ourselves the good and the bad parts and how they can sometimes even compete with each other for expression, and how they interact, blend and separate. As a result of this awareness, I do think many Fi doms consciously choose a siding with the light, to ensure the balance biases to the good. Yet we realize that sometimes tapping into the painful, messy bits can lead to .. the good (for lack of a better word). So, very messy stuff indeed.
Why the word 'surprisingly'? Because it's logical Fi doms should want to 'be' good? That our ethical awareness should cause every Fi dom to choose ... something that expresses in a morally positive way?
And as to 'tremendous acts of evil'? What are we defining as evil? Do you see Fi dom types as being so wired to positive values that it's difficult to conceive of negative expression? For example, I could be righteously energized to expose the corruption of a company (and I probably would) - does the tearing down of that make it evil? If that company employed hundreds of people who now don't have work? Someone could think that an act of evil. Does that make it so?
So, help me out with evil. Not holding the door? Mass murder? Where are we on the spectrum? Do you see a spectrum?
I like the following (link to page this is from at bottom of indent):
Fi has two forms, because it is an ethical process and a rational process. All rational processes work via dividing/contrasting. This is the essential element of judgment: to discriminate/differentiate. Because of this, as explained in this thread, Fe has a polarity to it in how it manifests externally. Fi also has two sides in which something can land according to its ethical frame. However, Fi is not a proactive process so this contrast won't be visibly exerted, instead it will be internalized.
Unless the worldview function (Ni/Si) has deeply cemented some truth such as "all humans are evil by nature", Fi will still be capable of making decisions on both sides of the contrast: to consider some things/people/actions noble and others not so much. However, if the worldview process has at least ingrained something moderately close to that such as "most humans are selfish, and incapable of true love" then the sort of ethical calls it will make will continually lean toward the darker contrast. A perpetual state of this can cause a perpetual sorrow on the face. This is because Fi accidentally radiates an emotional aura according to how it judges things internally.
What I've called an "innocent" Fi in prior posts is the willingness of Fi to be unguarded. It is typically Te which is expressed as a safeguard for Fi. Because Fi itself is not a proactive process, yet it is an ethical judgment process, it will implement its ethical judgment via Te. Thus an Fi that is either afraid of the world or jaded against it will come off as almost entirely Te. But if this type is a high-Fi user, then the Te will not be able to fully conceal the "pained" sorrow that will also lie behind the hard shell of Te.
As I've posted elsewhere, an example of an unguarded/innocent/seelie Fi would be this young Evanna Lynch. Her footage starts at about 2:00.
FiNe Evanna Lynch Video Here
Notice at 2:11 how she shrivels back into herself, in a very giddy and bashful way. This is how Fi looks when it trusts the environment and people. Because Fi is a compass process (Ji) it does not lie to the user. It's stances are essentially their identity - just as I cannot fool my Ti when it sees a contradiction, even though the most I can do is ignore it for a while. This authenticity makes it so Fi cannot lie to itself about how it feels about an ethical stance/position. But unlike Ti, the stance Fi takes on a position also affects the user emotionally and in their countenance. A sullen/pained countenance is the result if it's judgments are chronically on the negative side of the judgment-divide.
An example of "unseelie" Fi, apart from Viggo, would be Kanye West:
FiSe Kanye West Video Here
This sullen/pained effect (unseelie) is more common when the user also has Ni/Se. And this is because the Ni/Se worldview is more stable, connected, and it is easier to maintain a perpetual opinion. Inversely, the levity and optimism that accompanies Ne alleviates this tendency to become sullen as Ne may open more positive possibilities/scenarios about other people, where Ni/Se wouldn't as easily. However, it really depends on the Fi's values, as an Fi may hold a value to be charitable regardless of the nature of those around it, etc.
- Fi Function Profile
http://cognitivetype.com/profiles/fi.html
I recommend a watch of the videos too.
And that link to Fe, wow it's pretty good too.
But I digress. Read that, process, and get back to me. We can dig deeper. Where we need to dig is into the enneagram as well I think.