Here's an idea: have the mother seemingly guilty, but, in a twist, though she's fascinated by serial killers and a psychopath, she's not the killer. How to do this? Simple--don't reveal the killer's identity until the end (basically, what Child's Play's original script did, as mentioned in the previous update). In Serial Mom OTL, we know she's the killer from the start (which is similar to Columbo's plots; makes me wonder if John Waters was inspired by that TV show)--we're just waiting for her to slip up (which is, arguably, part of its charm). So, have it be revealed that one of her friends and family is the actual killer (amusingly enough, her son in the movie was played by Matthew Lillard--who was one of the killers in Scream, which could be considered a smart slasher in TTL--and her daughter was played by Ricki Lake; having the original Tracy Turnblad (she was Tracy in the original Hairspray, which was also directed by John Waters--she was a member of his production posse for years) turn out to be a serial murderer would be quite shocking, IMO)...