The reason is: when you put someone's else DNA on trial (trying to understand and implying that DNA + environment controls the behaviour) you're putting God's creation on trial.
Controls, no. Influences, yes. We have free will, but we also have a nature with various tendencies. Some people are more prone to anxiety or depression or alcoholism or whatever due to a combination of their genes and their life experiences, etc. There's absolutely nothing even close to heretical about acknowledging that.
My understanding of Catholic teaching is that God’s grace is sufficient for all men at all times to reject sin. Period. Though one of the punishments for repeated sin is to be, in a sense, for a time, left in your sins.
I think there are two separate issues being argued over:
1. excuses put forth on behalf of the perpetrator, and
2. quick expressions of charity toward a repeat rapist of children.
For some others and myself there are no adequate excuses, no factors that ameliorate the guilt. Who better than someone who was raped as a child knows the ugliness and pain that he is inflicting on another innocent? As stated above, God’s grace is sufficient for each and every man. On the practical level, our criminal system is rife with that approach and has in too many cases gutted the application of justice.
It would be easier to embrace the expressions of charity toward the rapist if they had not been accompanied by the excuses. And timing helps. If we had fully processed the horror of the sin, fully mourned over the loss of innocence and destruction of the children, seen justice done in the form of a sentence that would forever protect the innocent and adequately punish the guilty, then most informed Catholics would likely be ready to turn a charitable gaze toward the criminal and pray for his conversion.