Question: When Uncle Buck is going to the party to get Bug, he is stopped at a red light and there are two parents in the car next to him. The father says, "don't go in there with that hat on, they'll kill you". Why would someone be killed for wearing a hat?
Answer: The Uncle Buck movie was made in the late 80s, near the end of the Cold War. And Because the hat resembles the Russian ushanka worn by Russian soldiers in that time, it can be said that wearing it might anger some, especially young Americans opposed to world events at the time.
Question: In the scene where Bucks brother rings him up to babysit there is a body in bucks bed. In the scene before Buck is at a bar but brings no one home from the bar so where did the the extra person come from?
Answer: You can't actually see a person, all you see is a human-sized lump under the covers. It's possible that it's just bunched-up blankets, some pillows, or even clothes.
Answer: It's never explained, but the best guess is the hat was an Ushanka hat which is often associated with Russia. At the time, there was a lot of Soviet Union controversy so being associated with supporting the Soviet Union could have angered many people.
Chosen answer: He's being sarcastic, as Buck says in the beginning some of his hats anger a lot of people, which probably connects to when his hat gets taken by one of the teenagers at the party.
Buck is actually referring to his aviator style hat in the earlier scene about people being angered by it. He's wearing the fedora style hat in the scene en route to the party. But I agree the guy was being sarcastic about teenagers.