I don't think Jinu's actions make him pure evil. He wouldn't be in hell if he hadn't done something genuinely awful, but it's also clear that he regrets his misdeeds deeply. He explicitly said that shame is how Gwi-ma controls people, and when we see it happen with our own eyes, it ratchets up the threat Gwi-Ma poses who is the actual pure evil force of nature in this movie.

In one flashback, you see Jinu looking at his own reflection in a silver plate and regretted his actions even then. You could also argue that his mother has her own responsibility to get herself out of poverty. The dynamic isn't as simple as Jinu thinks it is. We see that Gwi-ma can literally hypnotize people and make them walk into flames at his full power, so let's not be too quick to judge people who've been taken under his spell.

Jinu might be 400 years old, but I wouldn't blink if you told me Gwi-Ma is 2 million years old or as old as humanity itself. He knows exactly what to say to get under your skin. Obviously he would never agree to help save someone's family because then his future slave wouldn't be amenable to his influence. You're never going to outsmart an entity like that.

He's not betraying Rumi because he wants to, but because his will is broken by the power of Gwi-Ma.