what Kira did was terrorism, and she freely admitted it on many occasions. Terrorism doesn't just mean "something the evil enemy does." It's a term that has a specific, objective definition, regardless of how pundits and politicians abuse it as a term of emotional rhetoric. Terrorism is is the use of random or excessive violence to induce fear and despair in a population in order to pressure them into abandoning an occupation, war, or other policy. It's generally a tactic used by a weaker group against a more powerful group that can't be defeated by force alone and thus must be demoralized to the point that its own leaders or populace decide to cut their losses and retreat. Which is exactly what happened in the Cardassian Occupation of Bajor.
In Khan's case, he used the tactics of terrorism for goals that were not actually terroristic. He bombed the archive in London, not to terrorize the populace, but to destroy the secret Section 31 facility it contained, and to lure the Starfleet leadership to a specific place so he could strike at them. Terrorism was just the cover he used, the feint that disguised his true intentions. This is something we've seen done in many movies, such as Die Hard