OK, I didn't read this as a DM punishment, but in re-reading, I can see where it can be seen that way. Which means it should be sending off warning sirens all over the place! Proceed carefully Halivar.
That being said ... I have run campaigns where characters turned bad! It can be an interesting development and can enrich the story. Even the temptation and ultimate dismissal of that temptation can be very satisfying. So, as is my fashion, I will go into detail on how I have dealt with this as well as my thoughts on how you might want to address it.
Situation #1 - Party is taking on a "mystery snake cult" in a forest. Manages to take out a supply caravan and take one person captive. Party rogue and bard take captive into forest to interrogate the prisoner. They do not speak a common language (I do not have a universal common tongue in my campaign). Miscommunication happens and the bard slits the throat of the prisoner. They dump the body and return to party. In my book, killing a helpless prisoner is murder and is an evil act. Party berates the rogue and bard. After the game, I let the two players know that I prefer to run my games as black & white/Good & Evil. Killing a helpless prisoner is Evil. Both players understand, but want to play the morally ambiguous role where anything you do to a potential enemy is "good". I explain that I understand the appeal of that style of game, but it does not fit with the Good alignment. Future "transgressions may cause an alignment shift.
I was a little peeved, but I had never sat down with the players and really defined what I expect from the various alignments. So, I wanted to reset some expectations.
Situation #2 - Party is in a necropolis and has holed up in an old tower to recuperate. I have just picked up Dragon #302 (December 2002) which has the article on selling your soul. The "Tainted" PrC that allows an evil outsider to start trying to tempt you to Evil. If you can resist, you can pick up some ... interesting ... powers. So, I decide to tempt the party. The house they are in was one of the last to fall to an army of undead. The adventurers that lived there made a last stand, but eventually died. As the party slept that night, each one was visited with dreams and visions of the previous occupants. Some were given visions from the perspective of a particular occupant. Others were given a 3rd party perspective. Each vision was different. I actually took each player aside to work through their dream so that nobody would have all the pieces. I wanted them to feel a bit isolated, but if they communicated, they would put the story together. Basically, some of the previous occupants fell to despair and accepted the assistance of evil forces. These same forces were tempting the PC's 353 years later. Each character interpreted their dreams differently and a few of them decided that forces of good helped the occupants of the house, even though they eventually lost. When a voice whispered in their head that they could have that power ... 3 players said yes!
I took those players aside and explained that they were being tempted and that if they agreed, the PC would immediately gain 1 level in the "Tainted" PrC. But, I wanted them to look over the PrC, think through the implications, and make a final decision. At that point, one of the players decided to back out. The other two decided to go forward. Great fun!
The player that backed out didn't want to risk his character concept. I was cool with that. He had invested a lot of energy into that character. One of the players that accepted wanted to do it because he felt his character was a bit naive and he wanted to play up the angst that the PC would feel once he figure out that he had let Evil have a grip on his soul. The other player (Playing the Bard from Situation #1) decided that his character wanted Power and was willing to accept it at any cost. The PC was confident in holding back any Evil influence since Power isn't Evil. What you do with it is.
In this case, I wanted to make sure that everyone was willing to go with the story for the sake of the RP. I wanted everyone to have fun and we all did. the key to both situations was that I communicated with the players so that we could eliminate any DM vs Player relationships.
I would encourage you to speak with the Player and clear the air. If the player did not see this as an attempt to turn toward the path of evil, offer an out so that the character can "see the light" and the party can resolve that issue in game. The gods work in mysterious ways. Perhaps the cleric walks into temple one day as the high priest is relating a fable about temptation and deception. The story sounds eerily similar to what just happened with the "paldain" offering to help assassinate somebody else. That could serve as the trigger that allwos the PC to realize the potential problem. If the player realizes that this is a temptation toward evil, and wants to play the fallen character, thenboth of you know that and can work toward that storyline.
Communication is the key. With good communication and understanding, you can really make the temptation a nice character developing experience. But, with poor communication, you risk turning things into an adversarial role.
I hope that long winded post helps!
