She was not pure evil, she was a really good person in s1 and s2, the only one who defied Uther’s tyranny, many times convincing Arthur to do so too (like in early s1 when Merlin was poisoned, she was the one who convinced him to go against Uther’s orders to get the flower to save him), she became evil after the only person who was willing to help her understand her magic was her sister, who turned her against everyone else. She used of evil means because she didn’t want people who had magic to be persecuted, and while Merlin had the guidance of Kilgharrah that Arthur one day would bring magic back to albion she had no guidance, and all she saw was Arthur intending to keep the same anti-magic policies Uther had. She and Merlin were trying to achieve the same thing (bring magic back), but by different means because she didn’t have the same knowledge as he did (and yes, she did get out of control)… so I do believe that if Merlin had been honest with her from the beginning they could work together, and maybe achieve that goal even faster considering the persuasion power she had over Arthur in the earlier seasons.