private HashMap<Player, Integer> killstreak = new HashMap<Player, Integer>();

    @EventHandler
    public void onKill(PlayerDeathEvent e) {
        Player p = e.getEntity().getPlayer();
        Player killer = e.getEntity().getKiller();

        if (killstreak.containsKey(p)) {
            killstreak.remove(p);
        }

        if (!killstreak.containsKey(killer)) {
            killstreak.put(killer, 1);
            return;
        }

        int KILLER_KILLS = killstreak.get(killer);
        KILLER_KILLS++;
        killstreak.put(killer, KILLER_KILLS);

        if (KILLER_KILLS == 5 || KILLER_KILLS == 10 || KILLER_KILLS == 15 || KILLER_KILLS == 20 || KILLER_KILLS == 25 || KILLER_KILLS == 30) {
            final int kills = KILLER_KILLS;

            Bukkit.getOnlinePlayers().forEach(players -> {
                players.sendMessage("§7Der Spieler §e" + killer.getDisplayName() + " §7hat eine §e" + kills + "er §7Killstreak!");
                players.playSound(players.getLocation(), Sound.LEVEL_UP, 10F, 10F);
            });
        }
    }
}