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. There's no justification for this because life is not sacred for its own sake; although of course labor in restitution is preferable when possible, it often isn't and sentencing them to life imprisonment is absolutely not preferable.
To be fair, this myopia can be somewhat justified for someone with Christian or similar beliefs about the afterlife, but it's also found in people who don't.
And they'll often go farther and say that not only is it wrong to kill a murderer, but it's somehow hypocritical. A hilariously inconsistent argument for anyone who supports fining thieves; and also circular since even if the person making that argument had some mysterious principle behind it, it's definitely not one any death penalty supporter shares.
It's not okay if you're the victim¶
Many people are much more opposed to retaliation if the person doing the retaliating is the one who was wronged; they'll smear it as "revenge" 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 unanimously supported if anyone else was the one doing it.
Not counting certain types of aggression¶
The difference between stealing and receiving a gift is consent. That's an absolutely massive difference that determines whether it's acceptable to take things from the person who did it. Naturally, 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 assaulting you).
But many people have a myopia where they only recognize certain forms of violence, for example assault but not theft, and think it's no different to punch someone for stealing (with full knowedge of your non-consent) your candy bar 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 can actually be seen as acting worse (I was frequently on the victim end of this in my childhood). Since to add rights is to subtract rights, it's crucial to not classify violence as nonviolence.
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, you don't lose your right to retaliate if they stop but leave you still harmed by their transgression and refuse to make restitution.
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 see that this is insane. 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 immediate threat doctrine¶
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 is 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 what they know, 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 goodguys to spare the villain, 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 absolutely insane and I don't understand why anyone thinks there's a worse scene in Star Wars. (Well, maybe the scenes in The Empire Strikes Back where Han sexually assaults Leia, but most people don't seem to have a problem with those either...)