I think he felt emasculated by her and her family. Although he was educated, he did not come from family money.
He mooched off her parents and her trust fund money. Her father founded a brokerage firm... FD was indebted to her father and owed him ~$1.7 million. He wasn’t like her dad, Hilliard Farber... They had the power and he owed them money. He was angry.
Sadly, I wonder if her father hadn't passed away, he probably
would not have allowed the financial downfall to get this far.
Her father was a bright, financially savvy guy. He knew how to
make money and how to invest it to make more money.
FD only knew how to take advantage of someone else's money
and it doesn't appear he was very successful at multiplying
that borrowed nest egg. for all his Ivy League education, it
didn't seem to help him much in the real world.
Once her father passed, and no more easy money for FD,
he decided to default on his current obligations to her family.
And it just went downhill from there. His personal image was
shot, he needed other women to boost his shattered ego.
JD was probably nagging him to pay back her family and he went
on the downward spiral to strike back and get revenge to those
he believed wronged him.
He totally lost control and the rest is history.