Zag has a seal to retrieve, so one of Marc's allies need to die here. Lilly is spoiled for choices really, she's got the possessor of her sister's eyes, the owner of her mentor's sword, and the asshole that had her village wiped out to pick from π
Though she's not killing Marc here for plot reasons I'm sure.
I'm intrigued by the idea that becoming a seal to hold back the demons was just a regular thing that happened. That would imply that this has been going on far longer than I'd realized or that whatever actually happened to Ashy when she became that seal was rather different. Possibly both.