What American was labeled a terrorist who is now a hero?

Martin Luther King Jr. He was certainly treated as a terrorist although the FBI probably thought of him more as an enemy of the state. Had his phones tapped by the FBI and subject to a concerted effort by the powers that be to destroy him. J Edgar Hoover supplied raw intelligence reports of King’s movements and other minutia which gave him top priority to Kennedy, and there were numerous briefings at the highest levels about Hoover’s belief that King was a communist who was set on destroying the US way of life. They even somehow got ahold of some sex tapes and sent them to King’s wife’s home accompanied with a letter which said that basically “all of the US would know him for the beast that he is”. On top of that there were a series of letters written by the FBI which attempted to convince him to kill himself.

And now he has a national holiday and is revered as one of the most important men of the 20th century. We then also named the FBI building in DC after Hoover.