I had a fun experience with this but it was due to a bug. The ONE time that I succeeded in forcing myself to play a bad guy in an RPG was ~~Dragon~~ Jade Empire for the XBox. I made it through the entire game, feeling terrible about being a dick to everyone and then, due to a bug (before consoles got patches), I couldn't beat the game on the Evil path. It would crash every time. So, I chose the good option in the last conversation and was able to beat it but got the good ending.

It was really annoying to put that emotional effort in and have it count for nothing. So, I just accept now that I'm going to pay a good character and either watch ending videos online or not at all.