Not "somehow". There's actually a theory behind it: when the government does it, it is (nominally) done according to the rule of law. In a democracy, that means that the people have condoned it, at least indirectly, and so it is less subject to the whims of a single individual.
You may not agree with this theory, and the theory may not actually apply in practice. But the distinction between individual violence and government violence is not completely arbitrary.
Not "somehow". There's actually a theory behind it: when the government does it, it is (nominally) done according to the rule of law. In a democracy, that means that the people have condoned it, at least indirectly, and so it is less subject to the whims of a single individual.
You may not agree with this theory, and the theory may not actually apply in practice. But the distinction between individual violence and government violence is not completely arbitrary.