(1) It's not your father (in-law)'s Corps anymore. After World War II, as I understand it, it was discovered that many soldiers did not shoot at the enemy but instead "went through the motions" to look like they were shooting them. So the military has upped the ante on indoctrination, to dehumanize the enemy, so that more of the latter get shot. So current broad generalizations about the USMC might be more apt than in the past.
(2) World War II -- in which I presume your father-in-law fought (and my own father and father-in-law fought then, both in the Navy) -- is often seen as the "good war," when most Americans thought that the cause was just. (I agree.) That again makes the Marines back then different from those now and might justify broad generalizations.
(3) Most importantly, broad generalizations should be seen as statistical statements. That is, they often don't apply to individuals. People should avoid the application of broad generalizations unless they make it clear that there are a lot of exceptions. Otherwise, it's nothing but stereotyping, bigotry.
(4) It's not the individual Marines that are the problem. Instead, it's the organization that they're in and the state that uses the Marines as a tool for imposing U.S. foreign policy. To quote General Smedley Butler.... no, I'll refrain, since he's quoted too much in these circles. -- Jim Devine / "There can be no real individual freedom in the presence of economic insecurity." -- Chester Bowles