You want the read moral of War Horse?: never trust a pale, thin man. Like the Gentlemen from Buffy they will steal your hearts (or your horses) and leave you for dead.
How do we know this? Well, pale, thin men show up twice in War Horse at the most inopportune times. The first appears in Chapter 11. He comes to Emilie's stables early in the morning, scouting for horses to, um, "borrow" for their cause. By taking Joey and Topthorn, this man pretty much signs Emilie's death sentence.
The second, and arguably more sinister pale, thin man is Monsieur Cirac, the "butcher from Cambrai" (20.10), a "thin, wiry little man with weasel eyes" (20.9) who wants to buy Joey from auction and make Jell-O and Joey burgers out of him.
Sure, these guys are just doing their jobs like everyone else in war. But these men in particular are opportunists, using the war to their advantage, regardless of the consequences. Everyone was probably pale and thin at this time (food wasn't exactly abundant during World War I), but a few men lived up to their cadaverous appearances and became true harbingers of death.