Valmiki Ramayan writes that Kaikeyi was always a slightly insecure, arrogant woman who was proud of her position as the favorite queen of Dasharatha. Although she was respectful to Kaushalya and Sumitra, there were instances where they had clashes too (unlike what TV serials generally show, with them being all lovey dovey). Kaikeyi did love Rama a lot, like her own son, but when Manthara poisoned her mind, she believed her easily because of her insecurity. She could not conceive the idea that Kaushalya might have more power than her in the palace if she became Rajamata, so she exiled Rama to protect her own position.
In Adhyatma Ramayana (and I think also Tulsidas), there is a far more ideal portrayal of Kaikeyi. TV serials generally base their depiction of Kaikeyi from this version, with her being very sweet, kind and caring towards everyone in the family, including her co-wives. In this version, Devi Saraswati sits on Manthara's tongue while she poisons Kaikeyi, or else Kaikeyi would never have believed her since she loved Ram so much.
In other versions, it is written that Kaikeyi exiled Rama not because she suddenly disliked him or had desire for the throne, but because she wanted Ravan to be killed. There is one story (which NDTV Ramayan showed in 2009) that Kaikeyi's maternal kingdom was once ravaged and plundered by Ravan, and that he'd raped many women too. To avenge the insult, Kaikeyi cursed him that her son would kill him. Since she considered Ram her son just as much as Bharat, she sent him on exile in order to kill Ravan.
One thing is true though. Kaikeyi cannot be compared to Ravan. She was not evil. She may have made some mistakes in her life, but she was never evil. Ravan committed many atrocities against women and sages, so it's foolish to excuse his actions by saying he was a "great" character and such a devout bhakt of Shiva. It doesn't matter. He still did many many sins.