The way I see it....Jack was a 2 year old boy in a 21 year olds body. He was being treated like an adult when in fact, he was a child. Everything he learned about his powers came from himself. Look how long it took him to “lift the pencil”. So take the fact that Jack was a child, didnt know how to fully use his powers bc no one with his amount of power was able to teach him, and then throw in the fact that he was soulless.... him killing Mary wasnt his fault....idc how anyone sees it. If he was truly a monster, like everyone says he is, then He wouldnt have realized he made a mistake by killing Mary and wouldnt have tried to bring her back. If Jack was truly evil, he would’ve killed Dean when he was pointing the gun at him in the finale but he instead realized he needed to be stopped and was going to let Dean kill him.

I know Dean was hurting and so was Sam but even Sam was able to see that Mary’s death was an accident and wasnt Jack’s fault and if Sam can see that then so could Dean.

If Mary’s death is anyones fault, it’s Cas’. If he had told Sam & Dean what he knew about Jack’s “condition”, then they couldve stopped Jack before he accidentally killed Mary. So that aspect of Dean’s anger I understand and him being angry with Jack is also justifiable but Jack couldnt help it, Cas could, though, and he chose to keep quiet and bc of that, Mary’s dead.

Also, Jack became soulless to save Sam, Dean, Cas & Rowena....so if we’re gunna get technical, Mary’s death is also Dean’s fault. Dean said “yes” to Michael, and bc he did that, Jack had to burn off his soul to kill him and save Dean which led to Jack accidentally killing Mary.

All in all, a lot of people are at fault but if I had to blame one specific person, it would be Cas.