Asklemmy

It's always a matter of degrees. The bigger the injustice, the more violence is justified to rectify it. It is in the disproportionality, in my view, where the problem arises.

Never forget that humans are just barely evolved apes. Sometimes a swift knock to the head is required to activate those neural pathways to discourage anti-social behavior. Not always, but also not never. Claiming otherwise is just self-aggrandizing moralization that people use to make themselves sound and feel superior.

this is where the mythological concept of sin sorta helps. So its a bad thing but basically you decide at what point doing the bad thing is worse than other bad things but you can't ever make it not a bad thing. You just accept its price at some point and its ultimately and individual decision and I don't think many will know until that moment. For myself I try to avoid it as much as I can but I don't know in what situation I will be driven to it.

I think it is hard to list all the situations but in the end it boils down to situations where both you personally as the person considering using violence and the average person could live with that decision in the long term. Both because that covers situations where you personally are a lot less or a lot more concerned with the consequences of your actions than the average person. And the average person instead of every single person because there are always some individuals whose views on the matter are a bit too extreme to be practical. Maybe instead of the average person it might make sense to use something like "90% of the population" but in the end you can't measure things that accurately anyway.

When folks are mean to service staff.

Striking someone that could cause lots of violence to others otherwise...
Of course violence would be the last resort in this case as well, in my opinion, but it would be the lesser evil.

Some people use violence to fuel their morbid curiosity.
Can it help an individual who delves into such topic through discussions and material?

Usually never.

Self defense comes to mind, but probably there are other examples.

All of the above that you listed

Violence, by definition, is an unjustified use of force. If a use of force is justified then it isn't violence.

For example, suppose you're walking across a bridge and you see someone about to jump to their death. So you run over, pull them back from the brink, knock them down, and sit on them. Have you committed an act of violence? I would say not.

On the other hand, suppose the person is just standing on a street corner waiting for the light to change. If you run over, pull them back from the curb, knock them down, and sit on them, that would in fact be an act of violence.