In principle, all ethics ARE feelings based, on some level. That's what it breaks down to. Why is it wrong to kill people? To steal? To betray? To lie? To torture? Because there is widespread consensus that we don't like those sorts of thing. It's feelings based.
But what if I'm in a bad mood, and I want to behave like a total asshole? I feel like it. It makes me feel good. And when I so happen to be a good mood, I'm really nice, because I feel like it. Can this be considered "ethics"? I'm right to do whatever I do, because I feel like it? So, it doesn't matter what I do; everything that I do is, by definition, ethical and moral. I think for something to be classified as "ethics" or "morals", it has to be static.
For me personally, I have a static sense of ideals. They do change, but only gradually. My actual BEHAVIOR fluctuates. Sometimes I behave ethically, sometimes I do not. Sometimes I'm virtually incapable of behaving ethically. I can be TOTALLY emotionally wrecked, and as a result behave like a moody prick. That I'm incapable of behaving well doesn't change the fact that the behavior is unethical. I may do a lot of things that are unethical, for various reasons, but that doesn't change my assessment that it's unethical. I might download stuff on-line without paying for it, for example. I'll park at a shopping centre, and walk across the street, even though parking's only supposed to be for that shopping centre. If the cashier makes a mistake and doesn't scan a particular item, and I end up getting it for free, I won't necessarily make the correction.
So, my beliefs are rigid, even though my behaviors fluctuate a lot around them.
So my new theory is that most people, including INFJs, don't actually have a strong sense of ethics. That doesn't mean that they behave any worse than people who do. Someone may happen to behave very ethically, because of other factors, like wanting to have a positive self-image, wanting to cultivate certain kinds of relationships, wanting to share in nice warm feelings (aka empathy). But, I don't think the motivating force is to be in line with a static sense of ideals. I don't think they have a model in their minds of what's right and wrong, and compare whatever's happening against that model. I'm looking at this more as a manner of operating.