When the caret is positioned at the beginning of a paragraph with hanging indent and the user press Backspace, the left indent is set to a negative value, the text goes out of the bounds of the document. 

Workaround:
Use the editor's method instead of the command:

private void RadRichTextBox_CommandExecuting(object sender, CommandExecutingEventArgs e)
{
    bool? parameter = e.CommandParameter as bool?;
    if (parameter != null && parameter == true)
    {
        DeleteCommand command = e.Command as DeleteCommand;
        if (command != null)
        {
            e.Cancel = true;
            this.radRichTextBox.Delete(true);
        }
    }
}