Do you have the ability to plan carefully, including for contingencies?
Can you remain calm and effective in stressful situations?
Do you have the physical ability to navigate terrain under adverse conditions, carrying equipment?
What do you have to lose? Underground activists are unlikely to be captured if they follow proper security culture, select targets carefully, and plan attacks to minimize risks. But if they are caught, the consequences are likely to be severe. People going underground should be prepared to wind up dead or in prison for life. They should also consider the loss of:
Do you have the temperament to compartmentalize your normal cover life from your underground life? Can you stay motivated and psychologically healthy without the support of an aboveground community, with only the camaraderie of a small cell? Can you keep information from people, including your closest loved ones and confidants, on a need to know basis? Read Hacker OPSEC With The Grugq and 73 Rules of Spycraft for some idea of what’s involved.
Can you find others you can trust, with whom to act? A single person can accomplish a lot, but a small group can be much more effective, especially if individuals have complementary skills.