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And this is why I'm gonna be giving a ton of feedback "to prevent party/guild/alliance/raid-mates and people on your friend list from killing you if you're red". They already can't flag on you, so why should they be able to kill you? This is one of the biggest exploits in the system, so I hope that Intrepid has already changed that and just haven't told us.
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BH flag only against the PKer, so PKer's friends would have to go red themselves if they wanted to help. So while it is possible that they'll try and help somehow, the BH would still have the upper hand.
Also, you gotta think about this in the context of the bigger picture of the game. How would a PKer appear while being in a group? The most likely case is PKs in a dungeon against another group, while the victim group did literally nothing to stop it. And considering that even Dygz said that his party would probably just fight back, I do think that most groups of people would fight back. But even if they don't flag up against the attackers, they'd at least immediately kill the PKer in response (cause it'd still be 7vs1). So in such cases a BH wouldn't even have the time to learn about the PKer.
Most other cases of PKs will probably be amongst solo players or some small group gankers, at which point a well-gear BH would probably easily deal with the PKer (well, relatively speaking). And this would only be more common if the corruption gain is not as huge as it currently seems to be.
Again, this is mainly coming from L2 experience. People would PK here and there, mostly for farming spots (though more often the target would just fight back). There'd always be some uberPKers, but those would usually pick a glass cannon build and try to kill weaker players, so literally anyone with good gear would wipe them easily. L2's corruption gain/removal values were laxer than AoC's are supposed to be, which is why people felt better about PKing more often. But L2 also didn't have a BH system, so all PKers either grinded off their corruption or were killed by normal players that happened to be near the kill spot.
I do think that with a proper design and balance of said design, Ashes could still have those normal somewhat common PKs, while also curbing some of the uberPKers because there'd just be more BHs around to simply prevent huge genocidal runs by those psychos.