If you haven't watched the movie Sneakers, I recommend it for research.
And think of this quote (supposedly from a Gerald Seymour book): "One person's terrorist is another person's freedom fighter."
There are always two sides to a story.
A protagonist is likeable if a complete person:
- give him hopes (besides the bigger goal he is working for as a cyber terrorist)
- let him love and be loved
- let him be loveable (cares for his elderly grandma, rescues street kittens... just not too stereotypical)
- give him a history (but prevent stereotypical background, like a sad childhood... )
- make him funny or let him make people laugh because he always nails it with his quick wit
- give him friends and enemies
- give him flaws and weaknesses... and strengths too
Sometimes it will be the small things that make a person likeable. The snarky comment when somebody is verbally attacked at the supermarket, babysitting the neighbor's son while hacking into the enemy's base so she can have a hour of uninterrupted sleep...
And there are character attitudes we always see as positive, like loyalty; kindness to children, animals, and persons in need; respect for the elderly; being funny without making bad jokes about vulnerable people; being reliable...
Take good and bad pieces, with a few more good pieces and try to mold a round and full person that is more than just the cyberterrorist with a talent.