Fascinating reading through the thread - I'll backtrack later, but just some initial thoughts.
There is a big difference between our dark sides and evil - they can overlap of course, but my Se clumsiness is part of my dark side, though there is normally no moral issue associated with it.
I think our dark sides have far more to do with our primary shadows than our secondaries. For INxJ's this is the arrogance of false certainty - the rightness that trumps all regardless. In both T and F varieties this can easily manifest in emotionally charged inappropriate aggression - overt in INTJs and passive in INFJs, though not exclusively so. Exactly how this plays out will depend, because there are several variant subtypes in each of these two MBTI types. The Enneagram is quite useful at teasing these out - so an E1 INFJ is likely to be different from an E2 or an E4 in terms of their shadow problems. I'm sure it will all come down to one or another variant of excessive rightness, or the emotional reaction to having to accept we are actually demonstrably wrong in something emotionally important to us. It seems to me that our tertiary plays a big role in justifying us in the shadows as well, with INFJs rationalising their false insights with convoluted false analysis, and INTJs with deep layers of misdirected righteousness.
What is evil? People of a variety of religious convictions will understand it as having an objective existence rather than being a human abstract construct whilst other people will see it as something that depends only on human perception. It's useful to consider the idea of the Devil, or of demons, if only because they provide a thought experiment to play with even for those who don't believe in them. The Christian Devil is said to have rebelled against God through the pride of false independence, and was chucked into hell as a result. From there he does all he can to piss us about and make our lives a misery both now and for all eternity if he can. He does this as an end in itself, rather than for any other reason.
I think if we subtract the theology from this we are left with evil as the deliberate harming of others, or of the world around us, for its own sake only. I came across an interesting contrast between the Harry Potter characters Volemort and Umbridge recently - it claimed that Umbridge was more evil than Voldemort because she had no other motive in damaging people than the harm itself. Voldemort on the other hand was motivated by the desire for power and the wish to live forever and he only harmed others as a means to this end, not as an end in itself. Of course Voldemort caused far more damage than Umbridge but that doesn't make him the more evil on this basis, it's just a natural consequence of the quest for power in the way he pursued it. I think Hitler was more like Voldemort in this regard, whilst Himmler was perhaps more like Umbridge.
