When a merged cell contains more than one entire column, the caret is in the merged cell and you chose the option to delete the current column, the action is supposed to delete all the columns in the merged cell. What happens instead is that only the first column is deleted and the merged cell remains.

Workaround:

            TableCell tableCell = this.radRichTextBox.Document.CaretPosition.GetCurrentInline().Parent.Parent as TableCell;

            if(tableCell == null)
            {
                return;
            }

            int columnSpan = tableCell.ColumnSpan;

            this.radRichTextBox.Document.History.BeginUndoGroup();

            for (int i = 0; i < columnSpan; i++)
            {
                this.radRichTextBox.DeleteTableColumn();
            }

            this.radRichTextBox.Document.History.EndUndoGroup("Delete Column");