What does Gene mean when he says "Phineas, you wouldnt be any good in the war, even if nothing has happend to your leg."

Finny is a poor candidate for military service because he is just too good natured. Finny loves people. Killing them isn't in his nature. Gene doesn't think his fiend has any evil in him; the kind of evil it takes to survive battle. Finny's injured leg is besides the point. Gene thinks Finny would rather share a beer with his enemy than kill him.