private void button1_Click(object sender, EventArgs e)
    {
        if (button1.Text == "Start")
        {
            isEvil = true;
            button1.Text = "Stop";

            Thread t = new Thread(StartTyping);
            t.Start(textBox1.Text);
        }
        else
        {
            isEvil = false;
            button1.Text = "Start";
        }
    }

    private void StartTyping(object obj)
    {
        string message = obj.ToString();
        int amount = (int)numericUpDown2.Value;
        Thread.Sleep(3000);


        for (int i = 0; i < amount; i++)
        {
            if (isEvil == false)
            {
                //////This does NOT work
                //button1.Text = "Start";
                //button1.Invalidate();
                //button1.Update();
                //button1.Refresh();
                //Application.DoEvents();
                break;
            }

            SendKeys.SendWait(message + "{ENTER}");
            int j = (int)numericUpDown1.Value * 10;
            Thread.Sleep(j);
        }
    }

private void StartTyping(object obj)
{
    string message = obj.ToString();
    int amount = (int)numericUpDown2.Value;
    Thread.Sleep(3000);

    for (int i = 0; i < amount; i++)
    {
        if (isEvil == false)
        {
            if (button1.InvokeRequired)
            {
                button1.BeginInvoke( new Action(() => { button1.Text = "Start"; }) );
            }
            else
            {
                button1.Text = "Start";
            }
            break;
        }

        SendKeys.SendWait(message + "{ENTER}");
        int j = (int)numericUpDown1.Value * 10;
        Thread.Sleep(j);
    }

    if (button1.InvokeRequired)
    {
        button1.BeginInvoke( new Action(() => { button1.Text = "Start"; }) );
    }
    else
    {
        button1.Text = "Start";
    }
}

for (int i = 0; i < amount; i++)
    {
        if (isEvil == false)
        {
            button1.Invoke(new Action(() => button1.Text = "Start"));
            break;
        }

        SendKeys.SendWait(message + "{ENTER}");
        int j = (int)numericUpDown1.Value * 10;
        Thread.Sleep(j);
    }

private void StartTyping(object obj)
{
    string message = obj.ToString();
    int amount = (int)numericUpDown2.Value;
    Thread.Sleep(3000);


    for (int i = 0; i < amount; i++)
    {
        if (isEvil == false)
        {
            if(button1.InvokeRequired)
            {
                button1.BeginInvoke( new Action(() => { button1.Text = "Start"; }) );
            }
            else
            {
                button1.Text = "Start";
            }
            break;
        }

        SendKeys.SendWait(message + "{ENTER}");
        int j = (int)numericUpDown1.Value * 10;
        Thread.Sleep(j);
    }
}