In episode 10 of Season 5 it was revealed that Hal Cooper did not have the "Serial Killer Gene", unlike Betty and Charlie. Throughout the episode, suggestions were made that the twins have predispositions to do evil.
Knowing about Alice's various psychological problems and how her dark past, what if she passed the gene on to her children, including Polly, who would have passed it on to the twins?
Before this episode, I had deduced that they had inherited from Hal, which would explain the "Blossom Curse". It is also possible that it is a recessive gene and Hal has not expressed it because of his mother. But did Glen really ignore that? I think not.