Not all evil is in the servants of Shai'tan.
As an organisation, the Whitecloaks genuinely believe that they are on the right side and are serving the Light. Their primary motive is not evil, per se, but much of what they do in pursuit of that motive is. For instance, they believe that all Aes Sedai are Darkfriends, and will often put more effort into working against the White Tower than into finding real Darkfriends.
The Whitecloaks, like the Seanchan, are not a wholly evil organisation; there are characters in both of these factions with whom one can, at least to some extent, sympathise. However, as a whole, both are responsible for a great deal of suffering, particularly among women who can channel. One could say that the answer depends on whether you measure evil by intentions or by results.
In my opinion, this is one of the greatest strengths of the Wheel of Time series: while good and evil do exist, it is often not entirely clear which characters are good and which are evil, and not all evil is concentrated in one place. Just as in the real world, there are factions which claim to be combatting evil while in fact being almost as evil themselves, albeit in a different way. Mordeth and the forces at work in Shadar Logoth are another primary example: in fighting the Shadow, they became as evil as the Shadow itself.
Disclaimer: Whitecloaks who are also Darkfriends, such as Jaichim Carridin aka Bors, are excluded from the above.
Robert Jordan has commented many times on the moral issues in his books, in fantasy in general, and in literature and the world in general. See item 21 here
, my favourite out-of-universe RJ quote (emphasis mine):
The reason for the popularity of fantasy, and the reason science fiction is fading in comparison, is quite simple, really. Increasingly in books and films, including science fiction but also in everything from mysteries to so-called "main stream literary" novels, the lines between right and wrong have become blurred. Good and evil are more and more portrayed as two sides of the same coin. This is called realism. People by and large want to believe that there is a clear cut right and wrong, though, and that good and evil depend on more than how you look in the mirror or whether you're squinting when you do. In fantasy, you can talk about good and evil, right and wrong, with a straight face and no need to elbow anybody in the ribs to let them know you're just kidding, you don't really believe in this childish, simplistic baloney. That seems to be less and less so in other genres.
Does that mean fantasy all has to be goody-goody on the side of right and black-as-the-pit on the side of evil. No. In my own work telling right from wrong is often difficult. Sometimes my characters make the wrong choice there. Sometimes they do things are quite horrific. But they try to find the right choice. This is the way I think most people see the world and their behavior in it—trying to do the right thing with the knowledge that sometimes you're going to make the wrong choice, and with "right" defined as more than simply being of benefit to yourself—and they want to read books that reflect this. Right and wrong are not simply different shades of gray. Good and evil are not simply a matter of how you look at them.