And even the most morally repulsive, malevolent person can still make a perfectly valid logically sound statement.

Their moral repulsiveness and malevolence do not magically invalidate their logic.

It's not about their logic holding consistency, internally or otherwise, but about their Ability to characterize certain things that their very character will undoubtedly taint or cast doubt on. That's what I think you're being obtuse about, you think that it's a matter of pure Logic or Reasoning, but it's a matter of Precision, Accuracy, and above all else a matter of Ability to Judge Correctly, not just to judge Logically.

The only "problem" being that you can never know "their character" or "motivations" etc, due to your EPISTEMOLOGICAL LIMITS.

Nonsense. I don't need to "know" their character. I only need to Judge them based on what I know about them, however limited that may be, I don't need to know Everything to surmise their character, it might be more accurate but it can be done accurately without considering everything.