Oh, Isabelle would be terrified if Alec killed Viola, intentionally or unintentionally. She may not like her mother, but especially as she gets older she comes to understand the importance of Viola's role in the family. Alec killing her would indicate a level of danger she hadn't expected...kinda like Viola feels about it if Isabelle dies, actually, except less morally horrified. Isabelle might try to console herself by saying at least she won't have to deal with Viola's abuse anymore, and try to romanticize what it means for her and Alec. She'd also almost certainly get off on it. But in the end, she'd ultimately see it as a dire negative.

For some more on "How would Isabelle take Viola's death?" try reading my good friend Ira's ( https://x.com/underground_ira ) fic on the subject, Family Portrait, linked at the end of this ask! It's not canon and was written by someone other than me, of course, but it's a remarkably good portrayal of the familial dynamics that would be involved in Viola's death and how Isabelle would feel threatened by that. The running current they wrote through the whole thing of Isabelle fearing that she'll be next is spot-on.

https://archiveofourown.org/works/56464024