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:
You could have killed someone!
The meaning is that, although you didn't kill anyone, there was a high chance of accidentally killing someone.
One common 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 "could have" is very different from "might have". In American English, we use "might have" to talk about something that maybe did happen:
A: Where's Paul?
B: I don't know. He might have forgotten that we were meeting.