It takes courage to stand up to a dictator, but even a small resistance can be effective. If the Korean people don't believe that resistance is futile, even with threat of harsh punishment, then it can be effective. There are some reports that protests and dissent are occurring in a slightly more tolerant environment than in past years, so that's a hopeful sign for those uncomfortable with the current regime.
Kin Jong Un's hold over his people is very strong at this point, but dissent has potential to take root in communities and then spread. With enough tension and unrest as a catalyst, it's possible that people will be able to form an effective resistance. Written messages seem like an insignificant step but it could lead to bigger things.
The North Korean government maintains a hydraulic control system with the country only being able to feed it self by imports from it's Chinese allies in return for exporting mineral and metals. Both imports and exports and controlled by the government which means that a rebel organisation would be unable to feed itself. Additionally seeing as North Korea is a nuclear state other nations would only be able to supply small amounts of supplies to any rebel origination. Additionally rebel organisations would be unable to coordinate and spread their message to people as the only forms of media and communication in the country are very strictly controlled. Additionally the people of North Korea are heavily indoctrinated many believing that the government can see their thoughts and will be able to find out any act of rebellion they attempt to commit.
North Korea has suffered under an oppressive regime for decades. The masses within the country are indoctrinated with anti-western propaganda. Dissenters are quickly snuffed out. Without an effective communication infrastructure with which to disseminate anti-government propaganda, a freedom movement will be slow to evolve. Furthermore, given the authoritarian nature of North Korean government, real strides for freedom will likely require bloodshed.
In North Korea, the government has taken all of the power away from the people. They cannot have weapons, they cannot own businesses, and they have no way in what they do with their lives. The government has weapons and a military and the ability to squash any kind of dissent.