Dagon was right about one thing: Kelly died during childbirth. However, nobody knows whether Dagon or Castiel will be correct about the future of this newest, most powerful Nephilim. There are lots of signs that point toward a being of unimaginable power, but good intention so far.
- He saved Kelly. He healed and resurrected his mother when she was carrying him. Yes, she died during childbirth, but this wasn't the devouring destruction Dagon promised. It appeared more that she was simply incapable of giving birth to a being like this and surviving.
- He chose Castiel as a protector. He actively chose to leave a Prince of Hell and go with an angel - perhaps because he wanted better care for his mother, perhaps because he recognizes Cas as an angelic being like himself, but he chose to help Castiel.
- He killed Dagon. It was the baby's power that allowed Castiel to smith Dagon - and in terms of good vs. evil, siding with an angel to kill a powerful demon is pretty straightforward.
However, all this can also be explained as self-protection. Jack needed to survive to be born. He needed Kelly to be cared for and protected. On the other side of the equation, Jack is incredibly powerful, and power corrupts. He is the son of Lucifer himself. He killed his mother while being born. There's a lot on either side, and his eyes aren't much help either. Angels have silver/blue glowing eyes when they use their powers, demons have red/yellow eyes (Lucifer's are red). Jack's eyes are golden - he is something entirely new.
Two Men And A Baby
As well as being an entirely new kind of power on the show, Jack is currently alone with Sam and Dean. The finale killed off Crowley and Castiel, Dagon is gone, Lucifer and Mary Winchester are in an alternate dimension, the Men of Letters are destroyed in the US, and God has gone on a soul-searching trip with the Darkness. The season ended with Sam nervously approaching Jack, and although he has a fully-grown body, he will probably need some help navigating the world - will Sam and Dean be the ones to try and raise (and restrain) the Nephilim? Originally they wanted to destroy him, but they have no way of overpowering him now (especially with the Colt destroyed and Rowena dead). Will they try and help him, or will he disappear, to become the big bad of season 13, causing destruction as he comes into his power.
The final question, of course, is whether Castiel's vision was true, or if it was planted by Jack to make Cas protect him. Or, is a world without suffering not all it's cracked up to be? The show needs some evil to fight, so could this be a chance to see Sam and Dean battling an evil that appears to be good at the beginning? We'll have to wait until season 13 starts this fall to find out.