Maryen still loves Kendra, and has the hope that one day she can be redeemed.
I know, that's why I said hated instead of hates.
And I don't think her resolve's weak, but that she's nearing the breaking point. All those urges she's ignored and repressed have only gotten stronger.
It's a good point, however, we still don't know a crucial part of the lore to make assumptions, most importantly how and when those urges appeared.
If she had them since her birth, then no, I don't believe that she just reached her limit after centuries of enduring it. If anything, it should work like a constant yet bearable pressure, that simply makes you more prone to submit in stressful situations. Like addiction that you can successfully fight when everything is good in your life, but will definitely go back to it at hard times.
If that red eye is something new for her, then it would work, but in that case the story itself contradicts it, because so far we can tell that Maryen has been having her episodes for god knows how long. They already made her a holy amulet, and even a prison cell to stay during recharge cycle. Those things aren't made in 1 day after all.
Also important thing is to learn why did they become stronger all of a sudden. I think that it might be because of MC, and not because he lets Maryen do... things, but just because he sticks with her constantly. After all, it can't be easier to keep your dark thoughts when subject of that thoughts is around you all the time.
