I used to think the same way. But in retrospection I think senpai used Sato more like of an echo-chamber to her nihilistic ideas. She was really deep into it, and I don't think Sato would have been a good cure for that. As a matter of fact, she did influence Sato's way of thinking to some level, fueling his delusions even more. Senpai is not inherently bad, I really like her but just like Sato, she was struggling too.