First off, I have to admit that it is an indescribable relief to learn that there are others like me who do this same thing. I tend to do the old doorslam on occasion, more so in my youth, and agree that it is incredibly immature. I am quite a bit older now, 36, and have a great deal more perspective. Thus, I do this with increasing infrequency. That said, I feel that I could still resort to this in times of great distress or feelings of constriction or emotional blackmail.
I generally did/do the doorslam for two reasons; a) the person has offended my sense of propriety to such a degree that I have no further use for them, or, more commonly, b) I wish to end a relationship without going through the messiness of actually confronting the person. I never feel guilty about the first reason. I did this to my own grandmother, because she was cruel and insane and I had absolutely nothing to gain from knowing her, and I knew that she would not change. I do not feel that this is an immature decision. It is from reason b) that I derive most of my guilt.
While nothing can excuse coldly cutting someone out of your life without informing them as to the reason, especially someone who cares for and loves you (and I would not try to excuse it), it does make a sort of sense where INFJs are concerned. For one thing, we want to please, and are incredibly averse to conflict. We are afraid that the person will blame us, and that her or his happiness depends on us. We wish to forgo any ugly scene that may transpire should we be called out. I have ceased doing the doorslam for relationships, and now attempt to at least inform the person. This rarely goes well, either, and seems to come out of the blue. but it is better than nothing. It would be difficult to convey to another type just how hard this is; the level of discomfort and anxiety involved. It is like pulling your skin off with your fingers. It feels like you want to die, right there, and that you probably would if it were possible and easily accessible. Far easier than dealing with it.
The fact is that, for us at least, it does not come out of the blue. So much of this is carried out within our own minds, over and over. I think that we often assume that the other person must somehow know. How could they not? (we think to ourselves). I realize that this is wholly unfair, and I am not attempting to justify, mind you, just describing how it is. It is more difficult still if there is intense emotion involved. In my case, I often become involved with people who are clingy, overly emotional, and quick to blame. It is so difficult for us to communicate how we feel, and equally difficult to imagine hurting someone who cares for us, no matter the reason. It is why we delay things of this nature for so long.
Once again, I am not attempting to rationalize this sort of thing, and think that it is quite immature and not something that we should simply excuse. Of course, if it is deserved, as in someone becoming cruel or harmful, then that is another story. Not everyone deserves an explanation, or a second chance.