Start a competitor to your past employer with the data as seed data. Never sell it. Never share it.

People are getting worked up over the ethics of this data. Every sales person in the world takes their client list with them when they leave or are fired from their employer. Sales people are poached by competitors solely because they have know the client list of the competition. Sales, biz dev, press officers, CxOs are all poached by competitors for their data and connections. The business world is a brutal place when you see it from the executive level. Ethics quickly go out the window when you deal with 8+ figure revenues and profits.