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In the case where the person says "I am leaving so don't PvP me".
The Corruption that would be gained from killing them anyway is supposed to deter them from killing you.
It's hard to understand from your perspective perhaps because you view people who don't want to PvP sometimes as being unreasonably demanding about when and how other people should leave them alone?
In that situation in BDO I was gathering. By going out to gather I understand that someone may want to attack me in PvP. In a game where they could benefit from killing me by getting my loot, I would understand that they would probably attack me anyway and I would accept that this is the choice I made because I was gathering.
If instead I was talking to an NPC out in the open world to see if they had anything interesting to say and got attacked I might also say 'hey I'm just here to talk to this NPC I don't want to PvP I don't have anything of value'.
If you kill me anyway you get corrupted. I didn't choose to put myself in a situation where you have a likely reason to attack me, I chose to do something that theoretically doesn't have a situation where someone attacking me has a benefit worth attacking me for unless they just don't like me. The chances of a random person 'choosing to kill me while I talk to this NPC and don't fight back' are lower.
Then I go out onto the ocean to explore, experience the ocean, roleplay, whatever. I meet another player and that player just wants to chat about the wind currents or whatever. A third player comes in and attacks both. Can't get Corruption.
The third player is not the person who did the incorrect thing here. The second player and I did. We entered a zone to explore when the 'purpose of the zone' is to be a battleground'.
But still, none of this means that we'd want absolute flagging as the only solution. I want that if I 'go near a pirate guild's fleet' and they consider me to be coming to get their treasure, they can attack me and become corrupted. I don't want them to 'not be able to attack me'. I want them to need to warn me or talk to me first so we can have an interaction where both sides are doing their risk assessment at the time.
I'm bothering to make this long post because you said you don't understand the nuance and I'm genuinely hoping that you actually want to understand it. Even if you don't agree with it.