Are there conditions under which the release of a worm, virus, or trojan horse would be morally justifiable?

Creating virus is ethically wrong but it is right in some situation where proper permission has been availed from the government or the same action is taken by the government authorities to capture the data of terrorist or other person who is planning to distract the nation.

So it is right to hack terrorist's computer to pull information about the destroy plan which they have and to trace their location.

It indirectly saves the citizen and nation. This is one another way to bring down criminal networks.