The answer to this is "yes." In other words, John Brown is both. Because he was on the side of the North and because the North won, we see him in a positive light today. However, the things he did would have been seen as terrorism if the North had not won or if he had done them in the cause of the South (like, say, John Wilkes Booth did).
The things that John Brown did in Kansas and what he tried to do in his raid on Harper's Ferry were very much like terrorism. For example, in Kansas, he killed men in front of their wives and children. We would certainly look on such acts as terroristic. In Harper's Ferry, he wanted to arm slaves who would then kill white people, not all of whom would have "deserved" it.
Yet, at the same time, it is hard (impossible?) to argue that he was not fighting for a worthy cause. Slavery was an evil institution that deserved to be destroyed.
So which is Brown? To be honest, you have to agree that he can be seen in both ways.
The answer to this question all depends on point of view. To abolitionists, John Brown was a freedom fighter. To pro slavery factions, he was a terrorist, someone who used violence or the threat of violence to achieve political goals. This same idea can be seen throughout history. Another great example of this is the Sons of Liberty back in colonial America. To Americans who opposed British rule, the Sons of liberty were great American heroes and patriots. To the British and their loyalist supporters, the Sons of Liberty were nothing but terrorists. It all comes down to whose point of view a person takes when looking at historical events.