Complex sentence: In the second sentence, the speaker is speaking with a certainty that Oswald "did" shoot Kennedy (according to the speaker, the protasis is false), and therefore the main clause deals with the counterfactual result--what would have happened.
Split into simpler sentences: In the second sentence, the speaker is speaking with a certainty that Oswald "did" shoot Kennedy. According to the speaker, the protasis is false, so the main clause deals with the counterfactual result, i.e. what would have happened.