“You could have killed someone!”

Your 14-year-old son took your car and drove it around. He was caught by the police. When you go to pick him up at the police station, you're angry and scared because it was such a dangerous thing for him to do. You say:

You could have killed someone!

(someone) could have (done something)

This phrase expresses something that maybe would have happened in the past if the situation had been a little bit different. It's a way to imagine something different happening in the past. In the example above, "You could have killed someone" means that, although you didn't kill anyone, there's a high chance of accidentally killing someone if a 14-year-old is driving a car around.

One use of "could have ___" is to express how dangerous something was that happened. Here are some common phrases:

Watch out! You could have poked somebody's eye out.

Oh my God, that was so close. We could have died!

Notice that this phrase is very different from:

You might have killed someone.

You would say this if you think that maybe your son really did kill someone. (This is what it means in American English. In British English, I think it might mean the same as "could have" but I'm just guessing. I don't speak British Englishvery well.)