Every company has its Snowden.
The problem is not surprising. It is symptomatic of two different, perhaps overlapping issues.
First, disgruntled employees may be a norm in a company where people are treated as afterthoughts, not assets. Revenge will be the resulting problem.
Or second, the problem employee is an underperforming narcissist with access to too much data. Better governance and security would limit the amount of damage this loose cannon will cause.
The solution is better managment--treat people better, but also compartmentalize their access to data and track their movements within IT systems. Anomalous access could be a warning of trouble to come.