The question would, under other circumstances, be just - but I don't care to justify myself here. Elsewhere, perhaps.

(My upvote unfortunately only brings the parent to 0.) This is the approach that I would have taken (and recommended). The author and context of a post matters. Coming from an author who is consistently arrogant and disrespectful changes the meaning of the question from curiosity to challenge. Justifying oneself in response serves to validate the challenge, signalling that such challenges are acceptable and lowering one's status. It is far better to convey, usually through minimal action, that the 'question' is inappropriate.

I would not have even framed the topic with the phrase 'be just'. You can explain the reasons motivating a decision (and even explain such reasoning for the purposes of PR) without it being a justification. It's just and interesting question.