Destroy the reputation of a company with 10+ years of experience on the market. And force dozens of other companies across the U.S. to apologize to their customers for leaking data.
Not to mention lawsuits and fines for such a leak.
1. Find the least-incriminating/reputationally damaging records within whatever quantum of the data you are prepared to look at
2. Make a website (with a landing page like for security bugs >.>), Tor site, pastebin dump or whatever else that seems reasonable
3. Publish 1-10% of the data (!)
4. Encourage the site to do the news rounds
5. Explicitly email the company to be concretely sure they know about the site (maybe even do the CC bomb thing, for extra overkill bonus points)
6. Provide contact info with clear indication you will promptly provide all info to an adequately verified third party
The leak should disappear within the hour presumably.
Naturally, brain-breaking levels of self-protection would necessarily need to be employed, to guard against incompetent/egoistic retaliation (and the systemic resources large organizations effectively own). Make the Protonmail address from a VPN over a VPN over Tor, for example. Or perhaps start with a voice-scrambled VoIP call before committing to a video chat. Good luck here, basically.
Your opsec is always poorer than you think it is.
Just send an anonymous tip to Brian Krebs or similar if you want to do the noble thing.
What?