Get as much information as you can about what made him so enraged about a benign comment from the caregiver. Once you figure out what made him want to hurt her, you can then help him with the reality that he must have distorted. It's possible that he took what he heard literally, and somehow thought that he was going to be put on the ceiling. This concept could be frightening to him. It's difficult to imagine that a child could distort reality to this degree, until we realize every adult has listened to a floor creak in a dark house and imagined burglars creeping around.
Strange things occur in the imagination.
One reason for Sean distorting his caregiver's intent could be guilt. If Sean has been rough or mean with his caregiver, he could be anticipating revenge. He may have understood a good-natured joke as her retaliation. Do not ignore this comment. Understand that your son is feeling upset when he speaks this way and encourage your caregiver not to take his words literally, but rather understand them as a sign of an upset child who needs help to calm down and feel better through talking things out.