There is no one distinction, not truly. Any Jedi could become Sith, and it has happened. But if you also mean of your previous conversations such a thing would make a Jedi into a Sith. Such a line crossed would mean taking a very steep path onto the darkside. Jedi Masters have walked such paths, and their apprentices often follow them. But doing so would no longer make them Jedi, and the other Jedi would search for this individual to bring them to justice.
If you are curious if what you speak of has happened, I know the universe well enough to say; probably. It wounds my heart to know but anyone can fall to the darkside like that. I have not known any who committed such a crime, and I would have found them if I did, but to say the Order is without fault and perfect would be untrue.