The problem with the scenario you are proposing is that, while it is a good motivation for leaving the church, it doesn't really provide sufficient motivation for a radical change in alignment to evil. I could see a paladin, presented with your scenario disgusted enough with "laws and Heirarchy" to change to chaotic good, but not evil.
You are presenting the classic Corrupt Authorities Scenario, which usually just leads to the character trying to expose the bad elements. To work for what you want to do, the character would really need not to be presented with just an immediate superior who is corrupt, but a totally corrupt heirachy, where there are no "clean" officials to turn to. However, in a world where magic and alignment are real tangible forces, it's hard for the corruption in the church to be deep enough to drive a character to evil without some pre-existing flaw or weakness to lead them down the path of evil.
Basically, I would say, don't make all the motivation external, which seems to be the main flaw in your scenario (IMHO). Evil works from within, either playing on flaws or seducing a person in to evil action. A lawful good paladin has to have a very strong drive to seek the kind of life a paladin offers and it has to be an equally strong force to drive them away from that.
If you want to bring the PC cleric in to the process, how about this. Since the character is "half-fiend" perhaps, the cleric could "curse" her or give her some relic that activates some of her heritage, so she gets a few HP back whenever she kills something. It could be an addictive sensation, the rush of power and health that comes from consuming part of another's life force. It might also cost her some HP or Con when she hasn't killed anything in a while. Something like that could easily drive her to be less and less careful about what she kills, eventually leading her to kill simply for the rush. At which point she is pretty clearly evil.
You are presenting the classic Corrupt Authorities Scenario, which usually just leads to the character trying to expose the bad elements. To work for what you want to do, the character would really need not to be presented with just an immediate superior who is corrupt, but a totally corrupt heirachy, where there are no "clean" officials to turn to. However, in a world where magic and alignment are real tangible forces, it's hard for the corruption in the church to be deep enough to drive a character to evil without some pre-existing flaw or weakness to lead them down the path of evil.
Basically, I would say, don't make all the motivation external, which seems to be the main flaw in your scenario (IMHO). Evil works from within, either playing on flaws or seducing a person in to evil action. A lawful good paladin has to have a very strong drive to seek the kind of life a paladin offers and it has to be an equally strong force to drive them away from that.
If you want to bring the PC cleric in to the process, how about this. Since the character is "half-fiend" perhaps, the cleric could "curse" her or give her some relic that activates some of her heritage, so she gets a few HP back whenever she kills something. It could be an addictive sensation, the rush of power and health that comes from consuming part of another's life force. It might also cost her some HP or Con when she hasn't killed anything in a while. Something like that could easily drive her to be less and less careful about what she kills, eventually leading her to kill simply for the rush. At which point she is pretty clearly evil.