It is always wrong to pick out some types of terrorism to excuse, while condening others. It's just as bad when people seek to justify atrocities carried out by suicide bombers as when they try to justify atrocities carried out by military units or weaponry.

Chomsky's definition strikes me as excellent: "Terrorism is the use of coercive means aimed at civilian populations in an effort to achieve political, religious, or other aims."   And it is never to be tolerated or accepted.