^The second person to take my obviously facetious comment about the casting of the prez way too seriously....FWIW, I actually thought this particular actor bore a stronger physical resemblance to Romney than the next fictional CIC out of central casting.
Given his crippled and nerdy appearance in '99, I suspect Stark's spurning of him was just the last straw on the camel's back - a lifetime of pain, torment and humiliation (in short, being bullied) finally ignited on that rooftop. Tony was the personification of everyone who had made Killian's life a living hell up to that point, and hence made the natural target for his revenge.
As I reflect on the movie, this is my main problem with it. A good villain can make or break this type of film, and this villain's motivation was too juvenile/clichéd. It was way too generous of Stark to actually take moral responsibility for his and his partner's actions. Being stood up/one-night-standed by a drunk billionaire playboy with a very public reputation for that sort of thing during a New Year's Eve party is no justification for conducting unethical research on other human beings and starting a terrorist organization. If either of them had just made an appointment with Pepper in 2000, a lot of innocent lives could have been spared. They were obviously psychos without any help from Tony.
I'm going to blaspheme here...I liked IM2 better. The story was a more relevant follow-up on the first film, dealing with the consequences of Tony's actions in that film. Here, the only hook to what had gone on before was the Mandarin, and they didn't even bother to drop a line of dialogue to make the connection with the terrorists in the first film. To the average viewer, this was for all appearances a new villain who came out of nowhere.