@Leuix—
The psychotropic effects of the belladonna drove the humans mad, including Laura and Flan. The cat was affected before the humans.
Belladonna was valued because it dilated the pupils (and most people see women with larger pupils as more attractive for the underlying reason that large pupils indicate interest), and because it can induce premature birth or abortion. (It is from the first use that it gets its name, “
bella donna” [“beautiful lady”].) But it is both toxic and intoxicating.
Laura, in her madness, thought that Remilia was responsible for the bloodshed; but even in her madness Laura continued to love Flan. Laura insanely imagined herself entering into a contract in which she would save Flan by trading her life to Remilia for that of Flan.
There wasn't really a contract; there was Remilia's desire to save Flan both as someone whom Remilia loved and especially as someone who had been loved by Laura.