"Kazit" indeed means "to spoil, to corrupt" in Czech (the -ti ending of verbs used to be used, but it is now considered archaic). Nowadays.
But then there's the legend of the three pincesses Kazi, Teta and Libuše, and I have a feeling a princess would not be called by a name that means something bad... So maybe there used to be an old Slavic word, that was transcribed as "Casi" in Latinised texts (as "Kazi" comes from Latinised "Casi" in the Kosmas
chronicles, the first text where the legend was written down).