public MyPlugin extends JavaPlugin{
    public boolean onCommand(CommandSender sender, Command command, String commandLabel, String[] args){
        if(commandLabel.equalsIgnoreCase("someCommandName")){
            Random random = new Random();
            int indexToKill = random.nextInt(Bukkit.getOnlinePlayers().length - 1);
            Player playerToKill = Bukkit.getOnlinePlayers()[indexToKill];
            playerToKill.setHealth(0);
            return true;
        }
        return false;
    }
}