> library(data.table)
data.table 1.9.4  For help type: ?data.table
*** NB: by=.EACHI is now explicit. See README to restore previous behaviour.

> library(reshape2)

> tmp = data.table(Grp=LETTERS[1:10], A=1:10, B=11:20, C=21:30)

> tmp
    Grp  A  B  C
 1:   A  1 11 21
 2:   B  2 12 22
 3:   C  3 13 23
 4:   D  4 14 24
 5:   E  5 15 25
 6:   F  6 16 26
 7:   G  7 17 27
 8:   H  8 18 28
 9:   I  9 19 29
10:   J 10 20 30

> subtract = function(DT) {
+   for (nm in names(DT)[-1])
+     set(DT, j = nm, value = DT[[nm]] * -1)
+   DT[, base::sum(.SD)]
+ }

> tmp[, list(Value = subtract(.SD)), by = Grp]
    Grp Value
 1:   A   -31
 2:   B     2
 3:   C     3
 4:   D     4
 5:   E     5
 6:   F     6
 7:   G     7
 8:   H   NaN
 9:   I     9
10:   J    10

> subtract2 = function(x) {
+   ct = length(x)
+   x[2:ct] = x[2:ct] * -1
+   sum(x)
+ }

> tmp_ = melt(tmp, id.vars = "Grp", variable.factor = FALSE)

> tmp_[, list(Value = subtract2(value)), by = Grp]
    Grp Value
 1:   A   -31
 2:   B   -32
 3:   C   -33
 4:   D   -34
 5:   E   -35
 6:   F   -36
 7:   G   -37
 8:   H   -38
 9:   I   -39
10:   J   -40

> sessionInfo()
R version 3.1.1 (2014-07-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United States.1252 
[2] LC_CTYPE=English_United States.1252   
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods  
[7] base     

other attached packages:
[1] reshape2_1.4     data.table_1.9.4

loaded via a namespace (and not attached):
[1] chron_2.3-45  plyr_1.8.1    Rcpp_0.11.3   stringr_0.6.2
[5] tools_3.1.1