Happicuppa, I can tell you from firsthand experience, loosing a parent, or both, will change someone significantly. Its mindblowing and painful, I can definitely see how Jack ended up like John
First off, I'm very sorry for you loss that must have been horrible and secondly thank you for enlightening us. I think that it's very hard in games and movies to transcend and show the viewers the emotional disposition of characters. Which I believe to be the problem with people being unable to feel sympathy with Jack or hating him as there wasn't much time to elaborate on his conflict or understand it. And with the contributing factor of having spent so much time on John's story, it makes Jack seem incomplete and unrelatable.