I love terrorism because violence is a good thing. I mean come on, who doesn't love a good action movie once in a while. All terrorism does, is provides us with an action movie to watch in the real world. Why is it okay to watch an action movie but it is not okay for terrorism?
Take the atomic bombing of Hiroshima for instance. If we had not bombed it we were planning an invasion on japan in 1946 anyway. If we had not bombed the Japanese would have used women and children and many other civilians as suicide weapons to counter our invasion. This would have resulted in the death of many many more civilians in japan. So the bomb although tragic and sad and yes very over the top, still saved hundreds of thousands more civilian lives. And this is killing for the greater good.
I do not think terrorism could ever be justified. It is morally wrong to kill civilians. They are innocent people who probably do not want the conflict to be in their country, or be a part of them at all. The kiling of innocent people is very wrong, and is never done for the greater good.
Terrorism is never justifiable when it results in the harm or death of someone, even if that death is for the greater good. There is a fallacy in the very premise, since the greater good is subjective to the terrorists themselves and there is no proof that any good will come from the violence.