Only when there's enough reason for them to do so. Sasuke never had any proper reason to turn evil (Talking about him betraying his friends and village BEFORE knowing the whole truth about Itachi).
The one character I would have been satisfied with turning evil is Naruto. His parents died saving the villages, his body used to imprison the Kyubi. Yet he was blamed for everything. He never had a proper upbringing, no proper parent figure, his childhood playmates were taught to stay away from him, treated as worse than trash by nearly everyone around him (with exceptions you could count on two hands). If he broke under all the pressure and decided to destroy the village, I would have supported him.
The reasoning that the Third Hokage did it to protect him from his parents' enemies is just stupid. He had a tailed beast strong enough to destroy countries inside him. A fact that nearly every adult ninja in the village knew about. Which means that the information could have leaked just as easily as him being the 4th Hokage's son if he was recognized as such. And let's not even mention him being part of the Uzumaki bloodline. They didn't even bother changing his last name.
Anyway, back to the topic xD
As I've said, as long as its warranted, I'd love to see a nice character turn evil.
Or just turn into an asshole and stop trusting/helping people after being betrayed/mistreated. Not necessarily evil, but not your typical nice main character either.
