"Terrorism, in its broadest sense, is the use of intentional violence and fear to achieve political or ideological aims." I believe every definition of terrorism will be similar. Nothing about that definition has anything to do with morality though. Terrorism can be acceptable. It's only people who have bought into the mainstream ideas that think terrorism is always wrong. I would say you need to re-evaluate your ideals if you think Hamas can be correct but terrorism must be bad. One of those does not follow from the other. Terrorism can be used for good, and there's no reason to think otherwise.

And sure, terrorism can be used to defeat terrorism. You can have counter-terrorist terrorists. I would personally argue they always are, and I think it'd be difficult to argue against that.