That depends on how, I think. To offer context on your question; I am no longer Jedi. I was exiled for breaking the rules, at least to the Council's perspective. But I am also not Sith. I still serve the Light Side of the Force.
My friends who left the Order did turn Sith, although not in the way of the previous conversation. Becoming Sith isn't one thing, it is a number of choices you make until one day you are no longer Jedi, but Sith. One cannot, in my belief, wake up one day to suddenly be Sith. It is choices.
The previous conversation would be a major one but to go back to your current question it is always how. It is a question of what makes a person decide to walk the road of a Sith.