what John whispered to Dean but I have my theory.
Oooo do tell, I love theories

.
OK the theories from the British jury ie me and my Supervisor at work are
1. The demon is Sam's daddy and all the other 'special' kids

. Actually this is the same theory we had last year

or
2. Sam and the others were sent to fight the demons. Neither of Sam's parents are his biological parents, just people to raise him. However Sam's mum knew all about this and was a special guardian to keep him safe until his powers kicked in. This would explain why the demon killed her first. It would also explain why he killed Sam's girlfriend - I think she was the replacement guardian sent to keep Sam safe.