I think it sounds like something people here would enjoy, although I personally would not.
If this is implemented as initially suggested, I would probably stop playing NTK. I think SamChris's spin on it is much better, although I would make a few suggestions on top of that.
1). The default, natural state of the PK flag is
2). If you turn your PK flag on, it *stays* on for a
minimum amount of time (same as in above points).
3). If your PK flag is turned on, you cannot enter
hunting areas where you could be killed by monsters.
4). You cannot flag for PK in hunting areas.
5). If you *attack* another player, your PK flag turns on
for a somewhat brief period of time (10 minutes?)
6). If you *assist* in the attack of another player, your
PK flag turns on for the same period of time.
What I mean by this is if you cast a supportive
spell on another player who has his or her PK
flag turned on, and it is effective (actually heals
vita, successfully dispels/atones/ASVs/whatever).
* * *
The above points should address several exploits inherent in a "world PK" system given NTK's other features, although I'm sure there are more.
I do feel this system would add to NTK, and give people who enjoy PK something fun and new to enjoy. It's just important that participation is *optional*, just like how participation in RP is optional.