It's because Sumire is a gambling addict.
Sumire is a good person at heart, but she's in a bad place. She failed and gave up on life, dived head-first into her vices (alcohol and gambling). Living life like that for some 5~10 years changes a person a lot.
Todoroki... Well, you see how she is, she's selfish, belittles others, takes advantage of people, sees gullible people as targets, refuses to see how her methods are wrong and instead blames her shortcomings on others...
They are both bad, but Todoroki is miles worse.
I mean, Sumire is immature and is shown to bully Souko.
But (and is a huge but), pretty much Souko says is more because Sumire is unaware of what she is doing and thinks is the way they "play" with each other, which Souko finds annoying as it's completely opposite to her character, but doesn't necessarily hate.
They live together, work together, and Souko seems to rely on her a lot.
Like it has been pointed out, she's an addict. It has lead her to heavily monetary issues, but she has also been shown to take responsibility for it, even when she hates it.
There's a huge difference between being immature and annoying, and straight up blackmail and exploitation.
When FemmeFatale was complaining about "the toxic industry that only uses you", that is who she was talking about.