You can give someone every advantage, increase their chances to the Nth degree, but you can't decide for them to choose good or evil. That's the essence of Free Will.
One Word: Judas. Talk about someone who had all the graces available, as he was in constant close proximity to God Himself, and the other Apostles, etc.
I've talked about this before on other threads. Parents can do everything "right", but none of it is a guarantee of anything due to ... free will. We know of course that Our Lord did "everything right" with regard to His formation of Judas.
With that said, there's probably also some psychological stuff that went into it.
So between environment, temperament, experiences, psychological tendencies / issues (some rooted in physiological causes, others not), and throw in the magical unpredictable exercise of free will ... and there are no guarantees.
Nevertheless, even in the case of Stafki, we have to keep in mind that if we had been born into his family, with his temperament, his psychology, had all his experiences, etc. ... would we have ended up any different? God alone knows, and this is the true sense of "who am I to judge?" We obviously judge WHAT he did as gravely sinful, immoral, deviant, etc. But God alone will sit in judgment of him with regard to his degree of sin and culpability, etc. So when Bergoglio abused "Who am I to judge?", it was in line with the same error that appears in
Amoris Laetitia, where some lack or diminution of culpability in the internal forum can be used to justify the sins committed in the external. While God can judge a diminution of culpability in the internal forum, even we can't do that for ourselves. Much less can we use a diminution of culpability for past sins committed to justify continuing to commit the same sin.
But, anyway, there's no reason to judge the Stafki parents as if somehow responsible for what their son ended up doing. We don't know. Only God knows. As Matthew said, it's entirely possible that they did everything right, but that their son just made a series of sinful decisions on his own that led to this, combined with some obvious psychological problems. When dealing with incestuous pedophilia, there has to be a psychological issue going on there ... whether he suffered from some issues related to his physiology or whether these psychological issues were caused by some past sinful decisions, God alone knows. Yes, I disagree with the Dimonds that all tendencies to immorality are simply the result of sin. We see a huge correlation that many sodomites, especially pedophile sodomites, were themselves abused earlier in life ... and that undoubtedly leads to psychological issues that contribute to their tendencies later in life. Some people, for instance, are prone to alcoholism, whereas others can drink to excess for years and not become alcoholics.