OK, but what I want to know is how you react to some person -whose belief system is internally consistent- who has just, say, committed a gratuitous murder. Are you committed to saying that there are no objective grounds to sanction him

There's an ambiguity here. A standard can make objective judgments, without the selection of that standard being objective. Like meter measurements.

Such a person would be objectively afoul of a standard against randomly killing people. But let's say he acted according to a standard which doesn't care about that; we wouldn't be able to tell him he did something wrong by that other standard. Nor could we tell him he did something wrong according to the one, correct standard (since there isn't one).

But we can tell him he did something wrong by the standard against randomly killing people. And we can act consistently with that standard by sanctioning him. In fact, it would be inconsistent for us to give him a pass.

if A, seriously pondering the nature of the compulsion behind mathematical judgement, should ask, "Why ought I to believe that 68 + 57 = 125?", and B answers, "Because it's true", then B is not really saying anything beyond, "Because it does". B does not answer A's question.

Unless A was just asking to be walked through the calculation steps, then I agree B is not answering A's question.

But if you want somehow to reduce this to subjective goals, then it looks to me that mathematics falls by the wayside - you'll surely allow this looks pretty dubious at least superficially.

I'm not sure I'm following the argument here. I'm saying that all normativity is hypothetical. It sounds like you're arguing there is a categorical 'ought' for believing mathematical truths because it would be very strange to say we only 'ought' to believe 2 + 2 = 4 in reference to some goal. So if there are some categorical 'oughts,' there might be others.

Is it something like that?

If so, then I would offer the goal of "in order to be logically consistent." There are some who think moral oughts reduce to logical consistency, so we ought act in a certain way in order to be logically consistent. I don't have a good counter-argument to that, other than asking to examine such a theory and wondering how being able to point out a logical consistency is going to rein in people with desires that run counter to it any better than relativism can.