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If that person is flagged (or you are both in a pvp event or open seas) - yes, you can kill them for free.
But an unflagged person is not a pvper.
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Uhm...
It's 90 seconds

And as I said, I prefer the current timer.
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And again. The "jump" you're doing, in this context, is against a green player. A, by default, passive player.
You are completely free to follow through on your "jump" and kill him. The system is not stopping you from doing that. The only thing it's doing is saying "if you kill a green player - the consequences are different from killing a purple one".
You ask me to see your pov, but you refuse to acknowledge that this game does not have a "free pvp" design. All player are non-combatants by default and return to this default state after 90 seconds of their last attack on a flagged/green player.
Your perceived unfairness comes from the inherent desire to be able to kill another player w/o a punishment. But any opponent can, at any time, decide that they've had enough pvp for the day and decline to flag back up. They won't be "hiding" behind the system or ragequitting (as Azherae put it). No player out there owes you a reciprocal flagging up at all times of your convenience.
I say "honestly no" to that question because that's exactly what I would say to a person who'd ask me "can't you see that him attacking me at any moment is unfair to me?" Both yours and this question are on the extremes of the pvp spectrum. You consider it a disadvantage that you can't keep your unfair advantage over a target, while another person considers it unfair that you even have that advantage.
The system is in the middle. Though it's not even the middle of those 2 extremes, it's way closer to the PK side, because you can still always attack another person and you're always free to kill that person. You just have to take into the account that there'll be consequences.
Also, you keep saying that you "lose advantage" due to this system, but how exactly is that true? You're free to keep up that advantage at all times. You don't need to keep using your strongest abilities on the target. You can just keep him at 25-50% hp until he either leaves, dies or fights back.
And if he does in fact leave, heals back and then comes back - how is that any different from him simply coming back after a death? And as today's showcase (and pretty much all the other ones) showed - none of our damaging abilities have smth like 1m++ cd. And even if the literal strongest ability does have that kind of cd - running away and healing up to full from it would potentially take ~the same amount of time. And this is w/o even mentioning that starting off pvp with it seems a bit silly imo.