microbiology - If a human takes antibiotics are all bacteria in the body killed?

No

There are several reasons why this might not be true, as Alexander has discussed. An antibiotic often has a molecular target that isn't present in all bacteria, it's extremely hard to get antibiotics to certain parts of your body, and some bacteria will be defended against a antibiotic attack by biofilms, resistance mechanisms, and sheer statistical probability.

That is not to say that many don't die. Indeed, one of the major causes of Clostridium difficile infection is that antibiotics kill most of your gut bacteria, allowing the somewhat better protected C. diff to proliferate, start producing toxins, and send you to the hospital with symptoms ranging from diarrhea to perforated colon and worse. That disease is a direct consequence of "Antibiotics kill some but not all bacteria in you".