Katerina lives in the past. As far as we can tell, she was born in a fairly prosperous and "respectable" family, with a degree of privilege. She just didn't have good taste in men. When we meet her, she is both mentally and physically ill, with no hope of treatment. She's also extremely abusive. She bullies Sonia into prostitution and beats her children. She says terrible things about foreigners and is confrontational and violent with people she doesn't like.
Still, Raskolnikov has compassion for her and doesn't seem to judge her. Perhaps because she is poor he doesn't place her in the same category as he does Alyona the pawnbroker, or others he considers victimizers. As with Marmeladov, our own compassion for her is strengthened by Raskolnikov's.
Incidentally, Dostoevsky and his mother both died of an illness similar to what Katerina suffers from – lung problems, possibly tuberculosis, which was not uncommon at all in the 1860s.