Stats

public abstract class Stats {
  private static MyPlugin plugin;
  private static Map<String,Integer> kills = new HashMap<String,Integer>();
  public static void setPlugin(MyPlugin pl) {
    plugin = pl:
  }
  public static void load() {
    for(String key : plugin.getConfig().getKeys(false)) {
        kills.put(key, plugin.getConfig().get(key));
    }
  }
  public static void addKill(Player player) {
    if(kills.containsKey(player.getName())) {
      kills.put(player.getName(); kills.get(player.getName()) +1);
    }else {
      kills.put(player.getName(),1);
    }
  }
  public static void safe() {
    for(String key : kills.keySet()) {
      plugin.getConfig().set(key, kills.get(key));
    }
  }
}

StatsPlugin

public class MyPlugin extends JavaPlugin implements Listener{
  @Override
  public void onEnable() {
    Stats.setPlugin(this);
    Stats.load();
  }
  @Override 
  public void onDisable() {
    Stats.save();
  }
  @EventHandler
  public void onDeath(PlayerDeathEvent e)  {
    if(e.getEntity().getKiller() != null) {
      Stats.addKIll(e.getEntity());
    }
  }
  
}