When good guys go bad

[Snappy]

I'm DMing an Eberron Pathfinder game that features a paladin who's getting involved with the players. I'm planning on him turning evil, betraying the players at a key point to achieve some controversial goal. I'm curious to know how other people have handled this situation. A lot of what I've thought about has been done several times before, and one particular player likes to nitpick when she thinks she knows what's about to happen, so any help would be appreciated.

[Xuldarinar]

Well, heres a question for you;

Do you want evil as in;


Blatant Evil: Hi, Im evil. Nice to kill you.
Disillusioned: Good and evil do not matter anymore. With these actions, at least I've something to gain from it.
Heavy Handed: I shall do what I must. This is a lesser evil for the greater good.
Deluded: No, -YOU- are the ones that are evil, and you must be struck down!

[Snappy]

Probably leaning toward disillusioned with a little bit bit of delusion. I'm thinking he could suffer a heavy loss, starting his fall, and he might blame the players for getting in the way of regaining whatever he lost (loved one, glory, etc...)