I don't think she believes humans aren't capable of evil things. But in this case he just presented her with something and she has a hard time of taking it face value. She thinks he could be exaggerating or colouring. If Jarol told her: "They locked us in a cold room and left for hours," she would believe. But she knows Karama could be telling her official propaganda or a lie. She had seen many "official lies" in her short time on Cardassia. She has uncovered one herself!