Difficult Selection

I am not sure what you want here.

Here you have the monthly Debits-Credits per Customer:

SELECT  ConsId,
        (DATEADD(MONTH, DATEDIFF(MONTH, 0, Billdate), 0)), -- Cast all Billdates to the month level
        SUM(Debit - Credit)
FROM    #Consumer AS C
GROUP BY ConsId,
        (DATEADD(MONTH, DATEDIFF(MONTH, 0, Billdate), 0))

The other option would be grouping sets, to give you the Monthly totals and the per customer totals:

SELECT  ConsId,
        (DATEADD(MONTH, DATEDIFF(MONTH, 0, Billdate), 0)),
        SUM(debit - Credit)
FROM    #Consumer C
GROUP BY GROUPING SETS((consid), ((DATEADD(MONTH, DATEDIFF(MONTH, 0, Billdate), 0)))) 

If this is not what you want, please provide an expected output for your sample data.

**** EDIT ****

Now that agron has explained what he wanted as the final output, this should do the trick:

Edit CET 15:11 - added temp Tally Table, because agron didn't have one - sorry!

/* 
    This is based on the BRILLIANT article from Jeff Moden on SQLServerCentral.com
    http://www.sqlservercentral.com/articles/Crosstab/65048/
*/

/*
    Find the start and end of the dates in the Consumer table 
    to create the dynamic cross tab casting dates to start of each month
*/
DECLARE @StartDate date, @EndDate date, @SQL1 nvarchar(4000), @SQL2 nvarchar(4000), @SQL3 nvarchar(4000)
SELECT  @StartDate = MIN((DATEADD(mm, DATEDIFF(mm, 0, BillDate), 0))),
        @EndDate = MAX((DATEADD(mm, DATEDIFF(mm, 0, BillDate), 0)))
FROM    #Consumer

/* Build the dynamic sql*/
/* @SQL1 is static */
SELECT @SQL1 = 'SELECT * FROM (
SELECT CASE WHEN GROUPING(ConsId) = 1 THEN ''Total'' ELSE ConsId END AS ConsId,'+CHAR(10)

/* @SQL2 is dynamic creating all columns per each month between @StartDate and @EndDate */
;WITH Tally (N) AS (
SELECT N = ROW_NUMBER() OVER (ORDER BY (SELECT NULL))
FROM sys.all_columns AC
)
SELECT  @SQL2 = COALESCE(@SQL2, '') + '       SUM(CASE WHEN MonthDate = ' + QUOTENAME(d.MonthName, '''') + ' THEN Total ELSE 0 END) AS [' + d.MonthName + '],' + CHAR(10)
FROM    (SELECT N,
                STUFF(CONVERT(char(11), DATEADD(mm, N - 1, @StartDate), 100), 4, 3, '') AS MonthName
         FROM   Tally
         WHERE  N <= (DATEDIFF(mm, @StartDate, @EndDate)+1)) d
ORDER BY d.N

/* @SQL3 is static too.  Note the final WHERE clause, which removes the Totals row from the results */
SELECT @SQL3 = '       SUM(Total) AS Total  
FROM (
SELECT ConsId,
                MonthDate = (DATEADD(mm, DATEDIFF(mm, 0, BillDate), 0)), -- Cast all Billdates to the month level,
                Total = SUM(Debit - Credit)
         FROM   #Consumer AS C
         GROUP BY ConsId,
                (DATEADD(mm, DATEDIFF(mm, 0, BillDate), 0))       ) d 
GROUP BY ConsId WITH ROLLUP
) CrossTab
WHERE ConsId <>''Total''
';
/* Run the query */
EXEC(@SQL1+@SQL2+@SQL3)