Castti Ch.3 Abandoned Village - Malaya was...

Completed chapter 3 of Castti and I'm left a bit without words about the events that happened. Trousseau's motivations to betray everyone was "I talked to some dude back then and he told me the truth", but they never elaborate much further on why he spread that terrible disease killing innocents left and right. He says that's the way to free everyone from suffering, but that's like...just...absurd and incomprehensible.
To add on top, we get the reveal that everyone else in the Eir's Apothecaries are dead trying to get Castti to safety. Including Malaya that was not someone who was angry with Castti, but purely her own imagination trying to remind her about the memories she lost. Now Castti is on her own and she needs to finish what she started.

Now next chapter is the grand finale with the battle against Trousseau to stop him from destroying a huge city while the coronation of the new king is ongoing. While I'm sure that Castti will succeed I'm wondering exactly WHO put these things in the mind of Trousseau. I'm starting to think again that there must be a central figure that is causing havoc around the country in all the various routes and they're all somehow connected, but so far can't still figure out the pieces.