what or who could make
Captain America this angry?

why, this idiot, of course.

said idiot calls himself
Flag-Smasher (try saying his name 50 times in one minute, and he
might just appear behind your back). his beef is that he hates any kind of nationalistic identity, and wants the world to operate as one country. if he were a stand-up comic, he might say he wants to put the "o" back in "country".
anyhoo, lame-ass name aside, he did start doing literally what his codename was, smashing flags in the U.N. Plaza, and crashing
Cap's press con to promote his 1-800-HELP-ME-NOW rescue number. being humiliated in public with one jaw-breaking punch,
Flag-Smasher (not sure how he got out of jail) led his organization
ULTIMATUM (understand that he's serious with this shit - he spent three weeks thinking about that name!) in hijacking a commercial jet, forcing Cap had to break his principles (only
Bucky killed in WW2, remember?) and gunning down a hijacker to save the hostages. check out the original
Mike Zeck badass cover -
Captain America, Uzis, Cold War, '80s ... awesome!

of course, Steve Rogers being Steve Rogers, when thrust into a situation that their survival preceded philosophical differences, he chooses to save Flag-Smasher's life, despite knowing the latter won't reciprocate if the situation were reversed. plus, they even had a nice little ... tender moment.

according to
Wikipedia,
Flag-Smasher ended up a ruler of his own country Rumekistan (pop. 1) - oh, the irony - before being assassinated by his own shadow due to boredom. no, it was the
X-Men who did it. the same
X-Men who now have their own Utopia.
co-created by the late great
Mark Gruenwald and
Paul Neary,
Flag-Smasher, Swiss-born, pink-visored, serious gay visionary, will always remain a source of infantile derision and humor among
Captain America fans forever.
-----
Captain America vol. 1, #312, 321-22