According to John McAfee (of McAfee Antivirus), it was a disgruntled employee in the IT department:
From the data that was released, it was clear that the perpetrator had intimate knowledge of the technology stack of the company. For example, the data contains actual MySQL database dumps. This is not just someone copying a table and making into a .csv file. Hackers rarely have full knowledge of the technology stack of a target.
There were countless ways the “hackers” could have ransomed the data. Instead, they only demanded that the website be shut down.
It’s sad that people who were exposed as AshleyMadison users are killing themselves. Even sadder for their survivng spouses.
Interesting aside: Hedge fund billionaire Dan Loeb had an account on AshleyMadison. When asked about it, he claimed it was for research, because he had invested in IAC (parent company of Tinder and match.com). His research involved creating a profile and checking for messages. That is some comprehensive due diligence. No wonder he manages a $14B fund.