I do not think a terrorist can do any good. Terrorist is a person, group, or organization that uses violent action, or the threat of violent action, to further political goals.
They use deliberate act of violence to create an emotional response through the suffering of the victims in the furtherance of a political or social agenda.
They also make use of psychological manipulation through warfare to the purpose of political or religious gains, by means of deliberately creating a climate of fear amongst the inhabitants of a specific geographical region. There is no good in all of these.
Terrorism is a tactic, and it cannot be "good" or "bad." We think of our founding fathers as revolutionaries and not terrorists, but that is simply our perspective because we are for the cause.
Terrorism, by definition, is supposed to instill fear in people. In some countries, terrorism is seen as a means to an end in order to achieve specific political objectives. In our modern society, we are facing the threat of religious terrorism. Right or wrong, terrorists definitely believe in what they are doing.