As everyone knows, Hughes' friendship with Roy ultimately becomes his downfall. Hughes was not a careless or reckless man, he was very attached to his own life and of course he had a family to return to. However, it was helping his best friend, albeit indirectly, the thing which led him to his death. I'm sure that while Hughes would never ever blame it on his friend or his goal, Roy does blame it all on himself.

Roy: Getting a double promotion for dying in the line of duty... Brigadier General Hughes, huh...? You said you'd support me from below. Just what are you going to do now that you've gone up higher than me? You idiot.

Roy has a thing for double-meaning phrases. He hides his concerns, fears and other emotions with sentences like the one above. I'd say it's a self-preservation thing. He probably thinks that if he doesn't speak up his worry or feelings, they can't be used against him and can keep people from knowing the real him and getting too close. Oh, how wrong he is.

     

Hughes' death, while devastating, is also a boost for Roy. He becomes more determined than ever, now that he's given himself another goal to reach: find out who killed Hughes and avenge him. It's obvious Roy is prepared to do anything for revenge of his best friend. We've seen he'd kill for it.

A curious thing that we notice after Hughes' death is that Roy seems to slowly start to open himself a little more to people he didn't open himself to before. Havoc and Falman are a good example, while he also starts sharing more information and actively becoming (even more of) an ally to the Elric brothers. And people start to notice this too.

Not much time has passed since the tragedy, but Roy is capable of acting pretty calm about his best friend's demise, if only because Roy knows that staying collected is essential when you're trying to save an entire country from tyranny. Of course, Roy will always start to lose his cool when confronted with someone who may be the one he's looking for. I say lose his cool, but not blinded by it, as proved by the Maria Ross incident. By now it's a given Roy is not likely to stop thinking straight however deep is the hole he's in.