I can see why you ask this question. If you read the whole work, the meaning will become much clearer. If you are wondering, these words come from an essay by John Aikin and Anna Letitia Aikin Barbauld, and it was written in 1773. I will add a link to the essay. Let me make two points for you.
First, there is an emphasis on human affliction. The author is saying that people respond to affliction and suffering with compassion and benevolence (kindness). In other words, this response agrees with human reasoning that has made a connection, a relationship, between the "moral and natural system[s] of man." We should define the "moral and natural system[s] of man" as both the philosophical and human view of suffering. This is why the author says that benevolence (kindness) toward human affliction "cannot appear wonderful" (cannot be something to wonder at) to people who reason in this way.
Second, even for the person who does reason or think in the moral and natural systems of man, he or she is able to see terror in a different way. More to the point, we all have this ability or propensity at times. The implication is that he or she would even be able to see terror in a delightful way. Here is a quote from the essay:
"But the apparent delight with which we dwell upon objects of pure terror, where our moral feelings are not in the least concerned, and no passion seems to be excited but the depressing one of fear, is a paradox of the heart, much more difficult of solution."
The point is that often we love terror because it does not have an association with our moral feeling, though suffering does have. On the one hand we shun terror because of the fear it produces; on the other hand we cannot get enough of the "apparent delight" of terror. This is a human paradox of why affliction receives compassion and benevolent kindness but objects of terror receive only fear.
That the expression of our kindly feelings, when exposed to human affliction or suffering, should give us pleasure, cannot be wondered at when one thinks about that relationship betwee the moral system and the natural system of man, both of which have connected a feeling of satisfaction with every action or emotion that produces general welfare.
But the apparent delight we seem to have when dwelling on objects of pure terror--when in situations in which our moral feelings have no association, and no passionate feelings are stirred up except for fear--is a paradox of the heart and it is much more difficult to explain [why objects of terror don't also call forth benevolence and kindness of feeling].