Most people would accept that it's moral for someone who's threatened with a gun by another to kill that other person, thus preserving their own life.
Many would still accept as moral the same situation, but where the assailant has a knife.
Many would again accept as moral everything the same, except instead of someone intent to kill with a knife, an assailant who only intended to rob.
Robbery is taking a thing against the wishes/desires/rights of another.
Some might argue that the government collecting taxes is robbery by that definition and thus they have the moral right to defend their property with deadly force. The government certainly will use force to ensure that it gets it's taxes. They won't shoot you but they will take away your freedom and the only way they can do that is with force or the threat of force.
At what point do you disagree with that line of thinking? Why? I'm not trolling you, genuinely curious.