It's not good, but it's a necessary evil. The major instances are life and liberty.

• Life: is someone threatening to kill you, trying to kill you, or just indifferent to highly dangerous consequences to your health as a result of their actions? Violence is necessary to stop that.

• Liberty: I think revolts by slaves against their masters, or people against their tyrants, are justifiable uses of violence.

As a socialist, I would consider the capitalist workplace to be under this category. We have no choice but to slave away for some rich oligarch if we don't want to die or suffer in squalor. We have the negative freedom to refuse, but not much in the way of positive freedom. There are bandaids in place to solve that, but the bourgeois constantly pick at that bandaid through lobbying and propaganda campaigns against social safety nets. Ideally, I would ask them to peacefully stand down, but more likely, they'd just send out the pinkertons like they used to. So, violence is our only realistic path to liberty.