"You guys can't hold the boss? F*ck, who's holding the boss' aggro? You guys can't get the last hit in??" After receiving the report, the guild leaders hastily asked in surprise.
Simply speaking, the monster belonged to whomever the monster was aggroed on. The items dropped would be protected by the system for that player. For the next 30 seconds, no one else besides that player would be able to pick up the items during that time period.
It all sounded very simple, but calculating aggro was actually quite complicated. Normally, the last hit counted for everything. It's worth several times more than a normal hit. This was easy to understand. After all, the last hit was the hit that took the other side's life. The other side would obviously have enmity towards the killer.