What would the order do if a Jedi would start to go through a «dark path»? Would they kick them out, or would they kill them?? And what if they went completely dark (not like Sidious, more like in a Vader way?)? What would the order do with them?

padawanlost:

They would
be given a choice: renounce their “dark ways” and return to the fold or be
considered lost. If someone was considered beyond “salvation” they would either
be exiled or killed.

“As Jedi learn more about the Force, it is not unusual for them to form their own theories about how and why it works. They question how, if the Force creates and sustains life, it can have a dark side. Some arrive at the conclusion that the Force is not divided into dark and light, that its energy is inherently positive, and that there is no “dark side” waiting to corrupt them. Time and again, this conclusion has been proved erroneous, and the Jedi who felt compelled to test the limits of the Force rarely perceived the dangers of their explorations. As they approached the brink of the dark side, some were rescued by other Jedi or came back willingly when they saw the error of their ways. Those who refused to renounce their mistaken beliefs were either exiled to the farthest reaches of the galaxy, or destroyed.”
[Ryder’s Windham’s Jedi vs. Sith: The
Essential Guide to the Force]

If exiled they would
probably end up in a prison like the Citadel. But, considering how the PT-era
jedi dealt with dark side users, they would most likely end up dead. Walking a way from the
other was only allowed when the person was not a dark side user. If you went
dark side, living peaceful away from the Order was not an option because the
order would consider them a potential threat to the galaxy. You would either
end up dead, arrested or on the run.