Kahane was also far from being left-wing. His real legacy and impact are unclear, as is much of his life. However, he was killed al kiddush Hashem, and if the FBI had looked past his radicalism and really investigated what happened, at least the first attack on the WTC would never have come to be.
He did not form the underground in any case; assuming the ShaBaK wasn’t in on that one too, it was the work of Gush Emunim activists.