You say that karma is working, in the case of B, to bring retribution for a past action, Y, which B had previously inflicted on another, and that A is therefore potentially free of guilt/responsibility for having spilled the hot water on B. A would just be the agent who is ‘used’ by karma to bring about this retribution. But I think the paradox of such a situation is pretty clear: that there is really no such thing as freedom in action, despite the appearance of free will which we would all intuitively think exists. It’s an unfree freedom, since karma informs or utilizes our motivations to bring about the actions which we otherwise think we are freely performing.
But I’d probably say that karma isn’t only being exercised against B through A’s act of spilling the cup, because we know that A deliberately chose to do it. I’d think that, if karma is a true description of agency, there would be a potential argument of infinite regression, since B’s act in his previous or present life also wouldn’t strictly speaking be his act; it’d be another act of retribution directed at some third agent, C, which was caused by C’s act of Z-ing. So it seems that there isn’t a way out of this circular argument with its intrinsic paradox unless we either at least admit to a rendition of the theory of compatiblism or reject the theory of karma outright.
Both these points are useful in that they illuminate the puzzle I was raising. I think there is another facet to the doctrine of karma that arises when you consider that in my original post, I had relied on a too-neat slicing up of events, their causes and effects.
To see this consider that in the original example, A‘s action does not have one effect alone; it has several. It hurts B, but it also spills water on the floor, wets B‘s clothes, perhaps breaks a glass and so on. These effects may in turn impinge on other agents; again, in each case, these accrue because of the affected agents’ pasts. So A‘s actions are not determined by B alone but by a much larger assemblage. Similarly, A‘s action is not the cause of B‘s injuries; it is one of the events–the availability of hot water, fuel for its heating, a glass to hold it in, and so on–that may be causally implicated. Each of those events’ effects contribute to B‘s injuries; each of them is due to B‘s actions in the past. In ascribing responsibility to A we rely on some ends to guide us: perhaps our society is interested in assigning tort liability and individual actors are the most appropriate loci of causal influence for it. But the presence of the other events implicated in B‘s injuries means that the deserts for B’s actions flow through multiple channels.
These points do not affect the ones made by theendlessknot3d above; the puzzles of responsibility and free will noted there still stand.