I think the sympathy was only supposed to last up until it's revealed to be aqua tofana. Janne is sympathetic to her to the end, because she still thinks that it's the husband's fault and not the wife's. However, we as readers, who see "what Janne can't", see that she's not all she plays herself out to be.
It also goes with the theme of the various chapters, where the central jewel plays symbolism to the story at hand. Just as how realgar and the other minerals in the box look beautiful, and how everyone first believed the bottle's contents was just a beautifying face lotion, the wife is supposed to appear in a positive light. And just like how you wouldn't know aqua tofana was poisoning you until it was too late, and how realgar releases invisible arsenic vapour, the wife is revealed at the end to be far more deadly than her appearance gives off.
I think ultimately it's supposed to remind you that things aren't always neatly binary, and that humans are complicated, messy creatures.