Luke: "Vader"¦ Is the dark side stronger?"
Yoda: "No, no, no. Quicker, easier, more seductive."
I think it's easier to become evil. I took a whole lot of things to turn Spike from evil to good and it was painful.
What does it take to turn to the bad side? Not much, I would say. A quick taste and you see that the bad side is easier.
In Willow's case it was different though. I wouldn't say that it was her true self that turned evil.
Her dark powers were unleashed by one important event and she lost control, her drive for vengeance got boosted.
I don't think that she really knew what was happening. But again, it didn't take much.
I would say it's hard to turn a hero into a villain without significant foreshadowing and lapses in integrity or quality of character. Once you believe someone is "good" it's going to be very difficult to believe and accept that that person is gone and give up on them.
A villain to a hero is more believable to me because most of the time we don't know what caused that character to be a villain in the first place. Therefore, it is somewhat easier for me to believe that a human being who was born innocent would seek to become good and innocent again than to believe that a noble, "good" person would want to become evil.