Hatedherexceedingly - Amnon's conduct to his sister was not only brutal but inexplicable. It would be easy to form conjectures concerning the cause, but we can arrive at no certainty.
Matthew Henry
Concise Bible Commentary
From henceforward David was followed with one trouble after another. Adultery and murder were David's sins, the like sins among his children were the beginnings of his punishment: he was too indulgent to his children. Thus David might trace the sins of his children to his own misconduct, which must have made the anguish of the chastisement worse. Let no one ever expect good treatment from those who are capable of attempting their seduction; but it is better to suffer the greatest wrong than to commit the least sin.
SDA Bible Commentary (limited)
Volume 1-7
15. Amnon hated her. The result was typical. Amnon was moved, not by love, but by passion, and now that his animal lusts were gratified, he had no further regard for his sister, whom he had so cruelly wronged.