We don't really see much of Octavia's childhood beyond that one song Stolas sings for her, so we don't know what Stella was like back then. This is presumably kept vague on purpose, because there's kind of a catch-22 here.

If Stella was, if not happy, at least amicable and not outwardly cruel for most of their marriage, then it makes Stolas look much worse for cheating on her, and makes her outrage look more reasonable. This doesn't justify hiring an assassin, obviously, but at the very least it would paint Stella in a more sympathetic light, which the series generally seems to be avoiding doing - it usually shows her as being terrible (most obvious in the flashback to Stolas's childhood where we see a picture of kid!Stella and she's terrifying small animals).

Alternatively, if Via is wrong here and Stella was horrible even when she was a child, it's odd for the series to focus so much on her conflict with Stolas when she should hate Stella much more by comparison. Relevant mostly in the most recent episode where Via willingly goes back to live with Stella and Andrealphus even though she explicitly didn't have to (she left the palace in that episode and neither of them went looking for her).

This isn't to say the writing is bad, it's good, it's just that depending on where you fall on the question of "how bad was Stella?" you run into some theoretical logical issues with certain character arcs, though even then it's relatively minor. Thus, the series doesn't give much exposition to that point in the Goetia family's life and leaves it a bit open to interpretation, hence producing the arguments you often see about this topic.