When the other has a poor (or non-existent) sense of boundaries.

As long as the issue is what's being attacked it's all a matter of good debate. However, the debate must be accompanied by facts leading to a sanely developing or developed opinion to be useful for all sides. Recognition of exhausting the debate is often a must to avoid progress towards personal attacks and maximize the usefulness of the debate.

I don't attack others views, I defend my own.

Defending your position has to do with providing your thoughts and feelings and any proofs on your position where as an attack involves name calling and making jabs at the other person or their stance. When defending yours, your focus is on yours. If you are attacking another's, your focus is on theirs.

well, they very often overlap. some of the best defences are attack.

I think the idea of justice is both theological and circular. A person who commits a crime or transgression typically does so due to a psychological scar or poor upbringing, not some predisposition to malevolence. A person typically is better corrected with positive reinforcement or (more difficult to achieve) a sort of forced sobriety. E.g. the quiet, nice guy telling you "you're kind of a bitch." Most crimes committed are due to a persons lack of resources or misconceptions of the world, not something worth "punishing."

Needs some context. For instance, I do not believe in spanking children. I see it as serving no real purpose and generally as an injustice. So do I interfere with a mother spanking her misbehaving child in the food court at the mall?

Situationally dependent.

I suppose that depends on how far one is willing to go.