foreach (Process p in Process.GetProcessesByName("explorer"))
{
    p.Kill();
}

var processName = "notepad.exe";

var connectoptions = new ConnectionOptions();
connectoptions.Username = @"YourDomainNameUserName";
connectoptions.Password = "User Password";

string ipAddress = "192.168.0.100";
ManagementScope scope = new ManagementScope(@"\" + ipAddress + @"rootcimv2", connectoptions);

// WMI query
var query = new SelectQuery("select * from Win32_process where name = '" + processName + "'");

using (var searcher = new ManagementObjectSearcher(scope, query))
{
    foreach (ManagementObject process in searcher.Get()) // this is the fixed line
    {
        process.InvokeMethod("Terminate", null);
    }
}

public void DeleteDirectory(string target_dir)
    {
        string[] files = Directory.GetFiles(target_dir);
        string[] dirs = Directory.GetDirectories(target_dir);
        List<Process> lstProcs = new List<Process>();

        foreach (string file in files)
        {
            File.SetAttributes(file, FileAttributes.Normal);
            lstProcs = ProcessHandler.WhoIsLocking(file);
            if (lstProcs.Count == 0)
                File.Delete(file);
            else  // deal with the file lock
            {
                foreach (Process p in lstProcs)
                {
                    if (p.MachineName == ".")
                        ProcessHandler.localProcessKill(p.ProcessName);
                    else
                        ProcessHandler.remoteProcessKill(p.MachineName, txtUserName.Text, txtPassword.Password, p.ProcessName);
                }
                File.Delete(file);
            }
        }

        foreach (string dir in dirs)
        {
            DeleteDirectory(dir);
        }

        //ProcessStartInfo psi = new ProcessStartInfo();
        //psi.Arguments = "/C choice /C Y /N /D Y /T 1 & Del " + target_dir;
        //psi.WindowStyle = ProcessWindowStyle.Hidden;
        //psi.CreateNoWindow = true;
        //psi.FileName = "cmd.exe";
        //Process.Start(psi);

        //ProcessStartInfo psi = new ProcessStartInfo();
        //psi.Arguments = "/C RMDIR /S / " + target_dir;
        //psi.WindowStyle = ProcessWindowStyle.Hidden;
        //psi.CreateNoWindow = true;
        //psi.FileName = "cmd.exe";
        //Process.Start(psi);

        // This is where the failure occurs
        //FileSystem.DeleteDirectory(target_dir, DeleteDirectoryOption.DeleteAllContents);
        Directory.Delete(target_dir, false);
    }