"I tried being good!"
"Wait, I'm actually evil as hell!"
hmmm. Writing seems a bit off there. Shock value for the sake of shock.
"Why bother being good if you're going to treat me like I'm evil anyway. I might as well just be who you want me to be."
There's enough evidence Seo had inner conflicts about morality even back before her 'memory loss'. It's my assumption she didn't like being the queen bitch but somehow felt stuck in the role. Faking memory loss was the perfect chance to restart, but nobody forgave her. And if she's going to get killed for it she might as well kill back. Not like she doesn't have experience, considering she used Michiru to nearly kill Miho. Which we still need to learn the reason behind.
I do however think Seo genuinely appreciated Satsuki's faith in her. And I believe she's intentionally twisting the knife with her, whether to stop Satsuki from feeling sorry or to lose herself in this persona and stop thinking about how fucked up a betrayal it is. It's also important to note the monologue from the beginning happens while Seo is unconscious. Don't treat it as exactly what she's thinking right now, but more of her general feelings.
Also... gut wound is usually extremely painful and actually takes a while to die from. Sup Author-san?
I'm assuming he got stabbed in the abdominal aorta, after last volume's arc about it, and Komachi would have given Seo plenty of stabbing pointers.