The reasons Sydney "adopts" John from abandobment (what can we forgive?)

Sydney's entrance and exit in the movie highlight the stories core theme.

In PTA's opening shot, we first get to see Sydney in full view through his reflection on the door of the restaurant, to show Sydney is always looking back at himself and reflecting on his past, stuck, traumatized, haunted because he has not yet given up the ghost of his past sin and seeks redemption through John. In the script, we learn Sydney killed John's father. He used to be a hitman or on some kind of job.

And so Syd is always reflecting, just like the ending when he enters the same coffee shop that he sought retribution in and is yet again haunted by noticing and then reflecting upon the blood stain on his shirt. He then decides to cover it up with his sleeve, learning to hide his past, saving himself.