Hello,
There's no info about how PvM System actually works on this game so I asked for official information directly to our devs :
The message with the killer of the Boss/Event is calculated based on the most damage that was done to the Boss/Event (in this case, our killer, he is written on global and also he win the credits for that kill).
Now, the loot (rewards) of the Boss/Event belongs to the PARTY (attention, not killer) which made the most damage on it.That means if I have a top damager in party who do let's say 10k dmg per sec and he is the only dmg'er from party, but there is another party that have 3x dmgers with 5k dmg per sec, in normal conditions (without PK/PvP and starting from the same moment) the party with 5k dmg per sec will win the loot of the Boss/Event while the top dmger on the Boss will be listed on Global and get the credits(our guy with 10k dmg per sec).
Now, when there is PK/PvP between parties, when a dmg'er is killed, his damage is still recorded as the most damage on the Event/Boss but because he died, the PARTY damage is recalculated without his damage (so loot is decided when the Boss dies based on the most damage made by a party after it recalculates the members who died).
I hope that everything is clear now.
Enjoy !