We don't even know how much Stella has apparently "abused" Stolas during their marriage. All we got of their marraige is the day the affair happened that night, and some family photos. Oh and them sleeping together in the same bed. It's really hard to get the full picture. The show says that Stella has just been a complete nightmare to live with all this time, but at the same time will show photographs of the family having a genuine happy moment.
View attachment 7709665
And this ain't a forced smile from Stella. She looks happy; they all look happy in fact. And I doubt Stella is pretending here because the show really wants to hammer in how stupid she is. Stella is not one to hide her true feelings; she's very open about how she feels at the moment. And Octavia doesn't hold any ill-will to her for her supposed perfect family life falling apart. She blames Stolas solely. And I doubt Stella was manipulating Octavia that entire time to be on her side because it shows that Stella doesn't plan things that far ahead for her advantage.
Now I'm sure in season 3 we'll see Stella being a terrible person to Stolas in the past, and softly retcon any sort of nuance about her; but as it is now, the worst thing that Stella did to Stolas before he cheated on her was throw a "not-divorced" party and make some rude comments to her friends about how terrible Stolas is at sex while most likely drunk. On the night when the affair happened. The violent and angry Stella we see now is because of Stolas cheating on her.