K.F.
I'm with the other moms that it really depends on his age and he may honestly have no idea what a serial killer is. You may want to call whomever your pedictrician is and discuss it with them to find out what they think. But definitely discuss it with your son to find out what and why he said it. Some schools are so strict now with the whole bullying thing and shootings that have gone on that they expell children for these types of things. Definitely get to the bottom of it. Best of luck.