Keep him separate from the other kids.
Several years ago, we had a "strange" child in care for a while. He would talk the same way. He was 4/5 years old.
If he was in trouble, he would sit in time out and tell my son and I all the different ways he was going to kill us. We ignored it and he would eventually smile, apologize and be as good as gold for the rest of the day. But it was very weird. He was with us for about 6 months. Had been kicked out of other daycares. His family moved and his mother quite her job to be home with the kids.
I saw his father last year. The child is now 9. His dad told me the boy is in a mental facility after having tried to hurt his little sister by throwing her down the stairs (she's OK) and trying to kill himself several times. The reason he gives for both hurting his sister and himself is
"Because I wanted to see what it was like".
