"Futility" is, as per American Heritage Dictionary,
- The quality of having no useful result; uselessness.
- Lack of importance or purpose; frivolousness.
Nothing I see in the quote above reflects on the nature of war, therefore nothing reflects on the uselessness or frivolousness of war--as these relate to the nature of war. In order to find a reference to the chararter of war in the quote, one must seriously read between the lines.
It is important to remember that Thackeray predates the critical theory that meaning in literature rests with each individual reader's impressions. It seems to me that,
(1) the quotation is not making an assertion about men's character but asking a question and revealing a mode of thought about men's character for the reader to mull over and draw a conclusion from ("I wonder ... cowards ..."), and
(2) the purport of the quotation is to draw an ironic comparison between the value of physical/military valor and the valor of other higher values and tacitly ask which one is more valuable. It may be that if one gets lost in "men's emotions" and the "futility of war," one overlooks the actual and universal point: which values are most valuable.