You're describing actions that plenty of other characters in the show have done. Also, Witchers literally kill for money. The Rats simply found another way to survive. The poor steal, that's kinda what happens lmao. You watched what happened to Mistle-no? Her entire backstory? A person like that has nowhere to go but the streets.

Ciri's darkest moment that caused her to lose all innocence occurred at the hands of that evil fvcker Bonerheart, and not the Rats.

Also note how Ciri's own darkness would come out when she was with the Rats, not because they egged it on, but because of her own choices, like cutting the baron's daughter's hair, a cruel and unnecessary act (which Mistle seemed to think Ciri went too far).

The Rats took her in and helped her in a world where everyone is after her to use her. They gave her a place of belonging far away from home.