I wonder what kept McDonald's enemies from shooting him on a streetcorner, drive-by style (or equivalent) instead of waiting to get him in an elaborate airline plot...
Who knows? Perhaps it was all planned out, and they were more afraid of taking out a charismatic American leader on American soil, than they were of murdering him with 300 other people thousands of miles away from America.
At the very least, I beleive that they knew where McDonald was 24/7 and they did not want him to ever achieve a higher office. Maybe KAL 700 going down was just a coincendence, or maybe the guy that gave the final shootdown order knew who was on the plane, and that pushed him over the edge to make the decision. We may never know.