Sorry to bump this but I definitely think the direction they were going for Mila was to say she was also degenerating. There's actually a little out of the way discussion you might miss but talking with a priestess in mila's temple after the gate is opened reveals that Liprica vowed never to have Celica stand in front of Mila.
Why would she vow that if Mila was completely benign? Her sealing the falchion WAS also likely an act of madness combined with the love for Duma. She knew he would destroy the two countries so if she was sane she wouldn't have done it. She unseals it later because just like Duma, she regains her sanity right before she dies. Which Duma also does so there's a set connection there. Finally, the characters during the first two acts point out how Zofia was suffering, how pirates were destroying the place all while Mila was at the temple not caring. All of that along with her psychotic slasher smile (why are we bringing up real life mental illnesses? This is a fictional visual work where facial expressions are what's meant to symbolize emotions and inner thoughts.) all point to her losing her sanity/degenerating as well.