Not that I feel the need to defend her because she's the reason I am what I am but, did you know Katherine as a human or as a feeling vampire? When I off my feelings I turn into a monster. When they're on I'll do everything I can to help anyone I can. By your logic, no one should have helped me when I first arrived here with it off.
Should we be wary of Katherine? Yes. But everyone deserves a second chance.