This question might be better posted on the discussion board because you will get a variety of answers and examples to your topic, which is very broad, but I will give you one example to start.
The most famous assassination in the 20th century of the United States was the assassination of John F. Kennedy in 1964. He was a beloved president and seemed to be the bright, young future of the United States. When he was shot in Dallas, the whole country fell into an absolute state of shock. Ask anyone alive at that time and they will be able to tell you where they were then they heard the news, much like anyone can tell you where they were when they heard about the attacks on the Twin Towers in 2001. The country was living through a a very scary Cold War, but American prosperity and the Baby Boomer generation were living a fine life. The senseless death of Kennedy shook people's confidence in all of that. The leadership of the country passed to Johnson, but things were not the same. The unanswered questions as to why it happened linger even to today.
Assassinations are sometimes so seemingly senseless that it takes people a while to process what happened, and the conclusions are never very satisfactory, which is ultimately very unsettling. Assassinations of public figures like Kennedy, Martin Luther King, Robert Kennedy, and even John Lennon make people question what most people want to believe is the essential goodness of humanity.