Myopias on violence
Most people don't see themselves as pacifists, but have many "myopias" or situations where they fail to recognize what's clearly a justification for violence. Here I'll compile and explain common myopias against violence.
A special case against killing
Many people think you can take the property of a thief or lie to a liar, but not kill a murderer, even when better alternatives like penal labor aren't available. This myopia can be justified for someone with Christian or similar beliefs about the afterlife, but outside of that, it's a mistake to assign value to life itself. Life is valuable because we can do things in life: we can find happiness, we can help others find happiness, we can find truth, we can express agency, etc. Life is not valuable if all of these things are removed from it, such as by life imprisonment.
It's not okay if you're the victim
Some people are much more opposed to retaliation if the person retaliating is the one who was wronged; they'll put the dirty word "revenge" on it and claim it makes you "no better than them". This shows up most commonly in stories, where the writer controls the whole dialogue and doesn't have to deal with anyone obnoxious pointing out that the "revenge" they're portraying as wrong would be seen as justified if only someone else were the one doing it.
It's pretty obvious why this one exists; it's much easier to criticize something that happens to be self-serving than something that doesn't. But not everything self-serving is immoral, and even if it is immoral, it has not at all the same status as aggression.
Not counting certain types of aggression
The difference between stealing and receiving a gift is consent. That's clearly a massive difference that determines whether it's acceptable to take things from the person who did it. This is true of any consent-violating act, and the retaliation doesn't have to take the same form (for example, you could steal from someone as punishment for assault).
But many people have a myopia where they only recognize certain forms of violence, for example assault but not theft (or recognize theft in a totally different plane of moral judgement than assault), and think it's little different to punch someone for stealing your cookie than to punch someone without that context.
This can lead to horrible situations where if you retaliate against a person who violates you in an exempted way, you are actually seen as the aggressor. I was frequently on the victim end of this in my childhood. Since to add rights is to subtract rights, it's just as crucial to not classify violence as nonviolence as vice versa.
To add rights is to subtract rights
Let's be clear: anything that forces someone else to be affected violates their consent, is wrong, and makes it okay for them to use a reasonable level of force to stop you and extract restitution. Things that are commonly exempted include restraining or touching someone without consent in non-injurious ways, "playfully" interfering with their use of their belongings, intentionally slandering them, breaking promises, or refusing to grant them privacy in their personal spaces.
Retaliation stops being different from aggression as soon as the threat is gone
Another common myopia is to exclude a past act of violence from consideration even when it was never resolved. Let's keep in mind that if someone steals from you but stops and apologizes, you still need your property back. Likewise, if someone violates you in any other way and leaves you still harmed and refuses to make restitution, you don't lose your right to retaliate just because they're not actively doing it anymore.
It's only okay when the government does it
Many people see a moral difference between a court sentencing someone and a vigilante inflicting the same punishment on the criminal. You don't even have to be an anarchist to dispel this one. Morality exists outside of the law and the government can't be justified in using violence if a normal person would be a criminal to use the same amount of violence in the same situation. The point of a court is to *find out* what would be the just thing to do and to *enact* it, not to *determine* what would be the just thing to do.
The immediate threat doctrine admits violence is okay if someone runs at you with a knife, but stops recognizing it as defense if the threat isn't as immediate; for example if someone's building nuclear weapons for a country to use to mass murder. For the purposes of defensive force, that person is a mass murderer. Whether they're *culpable* is separate and depends on other stuff, but you can kill someone in defense even if they're only about to kill innocents by accident.
Another and worse case is when the immediate threat doctrine exempts the people *at the top* of the violence chain - that is, it's okay to retaliate against the person trying to hurt you, but not the person who orders them to. In stories, it's very common for the designated heroes to spare the villain after killing mooks, or for someone to be portrayed as wrong for not doing so. Return of the Jedi was one of the most egregious examples I've ever seen, where Luke refused to kill the Emperor because somehow that would be turning to the dark side. That was insane and the worst scene in Star Wars. (Well, maybe the scenes in The Empire Strikes Back where Han sexually assaults Leia are worse, but most people don't seem to have a problem with those either...)