I have a hard time playing evil as well i couldnt do it in kotor ill try and play evil in alpha protocol in my third or fourth playthrough.
It's probably because you think "yeah, I'll try evil this time" and then as you play most games, the "evil" choices are usually really psychotic stupid choices rather then evil so you end up not using them.......
i think if you play evil you usually end up with cooler stuff and more money.