I consider it more of a challenge to play with a 'good karma', in Fallout 3 terms. But sometimes, I really just need to blow off some steam, so I save and go on a killing spree, the only problem is if you accidentally save over the first save, and then your karma rating has dropped big-time permanently - that's happened to us all, I guess.
If I ever see anything I really want or a lock that needs picking, I do it, but then do something goody-two-shoes to make up for it, such as give all my purified water to the sick guy outside Rivet City
P.S. Sorry putting that all in a Fallout setting, but it seemed like the easiest way to explain the complex demeanour of my character in any RPG.