Part of it is him just not wanting her to get what she wants out of spite.
But although Iustirho is massively self-centered, even he recognises that letting Zasaroja have free reign to do whatever she wants to the world all but unopposed would end in disaster - especially after seeing what just being exposed to Meleuma did to Yaldukra; even having stable chimeras being free to move around could be seen to be dangerous. In Telwas' ending path, you also see that wanting to keep Liesefranche safe is another major reason why he would consider dying or being sealed.
In any case, thank you so much for trying the game - it means a lot that you're looking forward to what will come next. It'll still be a while before anything can be shown for it, but hopefully the first teases / previews for it will be ready sometime in the second half of 2020.