I code PvP servers.

command /ffa:
    permission: ffa.join
    trigger:
        if {ffa::players::*} contains player:
            send "&cYou are already in the arena."
            stop
        add player to {ffa::players::*}
        clear player's inventory
        give player diamond sword of sharpness 2
        teleport player to {ffa::spawn}
 
on death of player:
    attacker is a player
    add 1 to {kills::%attacker's uuid%}
    send action bar "&a+1 kill" to attacker

public final class KillListener implements Listener {
 
    private final Stats stats;
 
    public KillListener(Stats stats) {
        this.stats = stats;
    }
 
    @EventHandler
    public void onDeath(PlayerDeathEvent event) {
        Player killer = event.getEntity().getKiller();
        if (killer == null) return;
        stats.addKill(killer.getUniqueId());
        killer.sendActionBar(text("+1 kill", GREEN));
    }
}