Luke Skywalker joined the dark side. It happened at the end of Return of the Jedi. Luke had refused every temptation, the power to rule the universe, the chance to reunite with his father. Then Vader read his thoughts and said “Sister. You have a sister. If you don’t join the dark side, maybe she will.”
Luke joined the dark side. For a brief moment, he gave in entirely. He was going to murder this thing that threatened his sister. Then he woke from the darkness. He saw that this evil thing, this dark monster, was just like him. When he realized that the darkness was in his own soul he was able to conquer it. All fear vanished in love, and he threw down his saber. No terror could shake or threaten him now.
Luke chose redemption, and gave his father the courage to choose redemption as well. Luke knew that Vader had good in him, but in his conflict with his father he realized that good guys are also corrupted with evil.
The Last Jedi was written by people who do not understand that evil is inside of us. The Resistance never gets into power in spite of their string of victories, because the writers think that good guys are good, and bad guys are bad. They think that if the “good guys” got into power that there would no longer be problems or conflict, but rather utopia. Because they do not understand that evil is within, they think that evil is only a problem if “bad guys” have power. But really, if the “good guys” got power there would be enormous conflict, because power corrupts, especially those who think they are good.
If the Resistance got power many things could go badly. For example, they would have to make concessions and deals in order to keep things running, which would cause discontent among their more puritanical followers, who would then rebel against the new establishment. Or if they did not make concessions, but enforced their vision of good without exception, their rule would be one of brutal oppression, which in turn would foment rebellion against them. In other words, they would have to operate as humans in a fallen world.
We are supposed to support the Resistance and think they are good because of who they are, not because they do good things. There is mass slaughter and civilian casualties on both sides. There is no longer a conflict between good and evil based on making the right choices. Being good is a static category, not an internal war against evil.
This moral flatness comes into contrast with the original series in the scene where Kylo Ren tries to get Rey to join the dark side. Unlike Luke, she is never tempted. There is never a chance that she will fall, because there is no darkness inside of her. She is not even tempted to use power for good. Her rejection is flat, and meaningless. She does not reject the dark side because she is choosing to do the right thing, but because she is good, and Kylo is bad.
But she is not really rejecting power, she is just refusing to share it with Kylo Ren. The Resistance does not mean to remain the underdog forever. They mean to destroy the First Order and kill whoever they need to until they are the ones in charge. That was the point of the casino planet: they will kill the rich, and free the slaves. The writers truly believe that if the good guys are in power, things will be good, because they are good.
This is what art looks like when people no longer believe in Christ. Christian morality has been replaced with the fashionable virtues of today, in this case, with the SJW mentality. Good is no longer an internal battle of repentance based on choices, but a static state based on identity.