It is eventually revealed that Jack's soul really is gone since killing Michael and he has lost much of his morals as a result. Jack brutally killed Nick for trying to resurrect Lucifer, instead of doing it in a more painless way and in a moment of anger, lost control and accidentally killed Mary Winchester.
Jack's descent into evil arguably began as soon as his power was restored, as, immediately after Michael's defeat, Sam, Dean, and Castiel looked noticeably frightened at the youngster's ominous power.
But without grace, his body degrades, and he eventually dies, getting to meet his mother in heaven. The meeting is short-lived, though, as Castiel is forced to trade Jack's soul for his life on earth so he doesn't end up in the after-life for angels.
Jack says he snapped and it was over quickly. He wishes it didn't happen. Dean is barely holding it together. He doesn't like that Jack is calling Mary's death, “the accident.” But he plays along and says they understand and they forgive Jack.
Due to Jack being the son of Lucifer, Dean originally viewed Jack as a threat that had to be stopped from being born. After Jack was born, with Sam unwilling to kill him, Dean would keep an eye on Jack, threatening to kill him if Jack ever turned evil.
Character Information
Jack Kline (b. May 18, 2017) is the son of Lucifer and Kelly Kline. He is the first and only known Nephilim to be sired by an Archangel. Jack was taken in by Sam and Dean Winchester who would teach him how to master his powers.
Jack uses his powers to heal Cas because the antidote obtained via the best plan of all time unfortunately doesn't work. When he tries to also heal Dean, though, Cas won't let him; he's confident Dean will heal (because he's Dean, and Dean is Dean) and Jack is being entirely too cavalier about his soul.
Supernatural 15x13 - Jack gets his soul back and says sorry to Sam and Dean for killing Mary!
Sam and Dean do their best not to snap, to pretend that they forgive him. And ultimately, they tell him about the fake spell and convince him to get in the box. They tell him it's their way to keep him “safe,” and a way to keep any more accidents from occurring. And much to their surprise, Jack gets in willingly.
After a soulless Jack kills Nick for trying to resurrect Lucifer, he accidentally kills Mary in a brief fit of rage. Mary is reunited with her husband in Heaven and is now at peace in a shared Heaven with John.
In season 12 of Supernatural, a brand-new character was introduced who became one of the most powerful beings in existence. This was Jack, a Nephilim -- a baby born from the union of a human and an angel.
Jack tells Sam that he doesn't know if he can open another rift, and that he needs to find his father, who will protect him. When Sam tells him Lucifer doesn't do that, Jack corrects him and tells him that Castiel is his father, because his mother told him that Castiel would keep him safe.
Instead, Chuck's ending will now be to grow old and get sick and just die, and no one will remember him. As for Jack, now that he is essentially the new God, he brings everyone back. Then he says his goodbye to Dean and Sam, who realize that Jack is Him. He promises to be with them, in everything and everywhere.
Jack was killed by none other than Chuck, a.k.a. God. Of course, Supernatural is no stranger to killing off major characters, and those characters are often known to be resurrected sooner or later.
Why Is He Afraid of Jack? Chuck appears to be afraid and apprehensive of Jack from the very beginning. Yet, Jack is just the child of a human and an angel, which would mean that Jack should be less powerful than Chuck. This is proved when Chuck easily kills Jack.
While trying to find a way out, the demon Belphegor possesses Jack's corpse and offers his help, blasting the souls out of the reanimated corpses. With an infestation of ghosts on the loose, the three reluctantly form an alliance with the demon.
Chuck provokes Sam and Dean to kill Jack but they won't do it and he kills Jack himself. He dies without a soul and is sent to The Empty. Billie resurrects him and he wanders the earth soulless with a plan to make a big sacrifice in order for Billie to put things to rights.
In Ouroboros, Jack absorbed the Alternate Michael's grace returning him to full power, although he had to burn off his entire soul in order to kill him. When his powers were restored, Jack displayed his full angelic wings to show he was himself once more.
Jack dies of his condition, leaving the Winchesters and Castiel in mourning. In an attempt to save Jack, Sam contacts angel expert Lily Sunder, now an old woman, who suggests that they can resurrect Jack and cure him with her magic which will use the power of Jack's own soul to sustain him.
Dean appears visibly saddened by Castiel's lack of faith in him. Castiel later returns in Two Minutes to Midnight and apologizes to Dean for not having more faith in him. Dean forgives Cass, and they reconcile.
Mary Winchester (Samantha Smith)
Targeted by Azazel, retired hunter Mary had the distinction of being the first character to meet her maker. The Yellow-Eyed Demon pinned Mary to the ceiling of baby Sam's room, slit her stomach, and eventually set her ablaze. Mary's gruesome demise forever scarred the Winchesters.
It is revealed that she was killed because she interrupted the demon as he fed his blood to baby Sam. Sam learns for the first time that he was fed demon blood, and that Mary knew the demon that killed her.
Before dying, John tells Dean, if he can't save Sam, he'll have to kill him, should he become evil. The fourth season episode "On the Head of a Pin" reveals that the demon Alastair tortured John in Hell for over a century, with John refusing the demon's offer to stop if he himself would torture someone else.
Castiel: Resurrected three times by God and once by Gadreel. He was resurrected another time by the Shadow and later by Jack.
While trying to cope with his impending death and make the best of his final year, Dean begins to start hunting the freed Demons. Meeting a seemingly helpful demon Ruby, Dean learns that he too may become a demon himself if he burns in Hell too long.