They didn't kidnap her with intent to kill or torture or worse. They were keeping her safe as ransom. That in and of itself is not inherently evil.
As someone pointed out: Kidnapping with no threat behind it is inefective. There has to be implied risk to NOT co-operating with the demands of the kidnapper, esp. true in a world with divination magic (where it's only a matter of time until they find the kidnapped, and are able to, say, Teleport Without Error people in to retrieve her).
I maintain that kidnapping, in and of itself, by it's very nature, is an evil act. You are forcing a person to do something against their will, almost assuredly to their detriment, puting them in danger, causing mental distress to their family and friends, and by the very nature of the crime you must be willing to take steps to ensure that these things happen. (IE, resorting to violence if the kidnapped one tries to escape.).
This is taken from the SRD...
Good characters and creatures protect innocent life. Evil characters and creatures debase or destroy innocent life, whether for fun or profit.
"Good" implies altruism, respect for life, and a concern for the dignity of sentient beings. Good characters make personal sacrifices to help others.
"Evil" implies hurting, oppressing, and killing others. Some evil creatures simply have no compassion for others and kill without qualms if doing so is convenient. Others actively pursue evil, killing for sport or out of duty to some evil deity or master.
People who are neutral with respect to good and evil have compunctions against killing the innocent but lack the commitment to make sacrifices to protect or help others. Neutral people are committed to others by personal relationships. A neutral person may sacrifice himself to protect his family or even his homeland, but he would not do so for strangers who are not related to him.
Now tell me. Of that, what does this act most closely fit? Lets se...
Altruism? Nope.
Respect for life? Nope.
Self sacrifice in any way? Nope.
Compunctions against killing the innocent? Nope.
Concern for the dignity of sentient beings? Nope.
Protect innocent life? Nope.
That pretty much covers the things mentioned for good and neutrality... now if we slide on down to "evil"...
Debase or destroy innocent life for fun or profit? (Profit, in this case). Check.
Hurting, Opressing, and Killing others? Yup.
Hmmm...
And let me state one more thing:
It does not matter the REASONS for this act, really. They could have had the best interests of the entire planet in mind. This
act was evil. Regardless of if there was malice behind it (I maintain that there was, by the very definition of kidnapping), regardless of the motives, regardless of the circumstances, the
act itself was evil.