Violence is not always physical. A threat can also be an act of violence. Trying to intimidate others or force them to do something by referring to a divine authority for instance can also be an act of violence. In the same vein protesting against an injustice can be an act of violence. Of course violence like this is sometimes justified but when it comes to physical violence I would argue that it is only justified if every other options have been exhausted. Otherwise it gives the other party again a justification (or at least a perceived justification) for violence, which leads us very rapidly down the spiral of violence.

Yep

Depends on what you are willing to put up with. We could have prevented a lot of soldiers dying by allowing Hitler to carry out his genocide.

And, it depends on what you mean by justified. Anyone who uses violence can give you a reason they did it. Are you advocating an absolute standard to determine which reasons, if any, are valid?

Only in self defense or to protect someone else.