Who knows?
I think it's possible that KP was an Eddie Haskell type. (Hopefully most of you know the type?)
Given how she described him as cleaning up, helping out and rocking the baby AND how clean-cut he seemed to try to be, at least on his FB page, I can see him pulling off that nice boy image for some. And then letting his nasty side out around his friends -- with the alcohol and the




and the lecherous fantasies. I highly doubt he ever let the true extent of his perversions out for anyone to see. So sure, maybe JA should have put two and two together, but he seems naive at best. And maybe grandma should have, but she may not have been seeing the whole picture.
Were there red flags? Probably. But the frog doesn't notice the water is boiling until it's too late, and neither did the Ammermans.