I think such rpgs are not designed for evil gameplay, because being simply evil does not make sense in 99% of cases anyway.
You would need a completely different plot designed for interesting evil gameplay, especially when it's not only "killing good guys" but interesting interacting, plotting and intrigues. Almost every game trying to enable a designated good and evil walkthrough would fail. Good to some sort of greyish is the best someone could expect (I'm okay with that), but interesting evil walkthrough which does not break the actual campaign is very hard to implement.
