I don't see how this was a simple robbery. The guy was not rich - just purchased the home from a foreclosure.
Don't think this has any relevance but purchasing a "foreclosed" home doesn't mean you're poor. Often people purchase them, fix it up and resell it for a nice profit.
However, I think those guys simply didn't like their superior and the fact he married "one if their own" - in other words I think they're black rasists. Of course, it's just my speculation and I hope the truth will come out soon.
However, no matter whether they're found guilty or not by the courts - Marines don't leaves other Marines behind. Neither do they ever forgive Marines who've killed other Marines. Those guys are marked for life...