If the player fell in the void after 5 or less seconds of being hit then that would count as a kill for the last player who hit that player. However, if he/she fell in the void after 6 seconds or up of being hit then that wouldn't count as a kill for the last player who hit that specific player.
Let's have a first scenario :
The game started and a random player hit another player using a bow on his/her island and didn't fall at all out and both two proceeded on their own businesses.
6 seconds later, this player who got hit by an arrow recently fell in the void WHILE MAKING A BRIDGE then the reward won't count since it's been 6 seconds.
Second scenario,
A player is busy putting his/her armor and suddenly a random player hit his/her to the void (the scene is probably fast enough that the player will take less than 5 seconds before hitting the void) (the random player who hit that player gets the reward).
Excuse any typos, hope you understand what I'm tryna to say.