Cyber vigilantism is a very dangerous proposition and can lead to inadvertent escalation to nation-state action.  It would be better to recruit and form a cyber militia that is integrated into the military and national command structure.
The Digital Vigilantes that Hack Back

Authentication protocols and the underlying cryptography is extremely difficult to get right.  When it goes wrong, it can leave major vulnerabilities.
Serious flaw in WPA2 protocol lets attackers intercept passwords and much more
Millions of high-security crypto keys crippled by newly discovered flaw

Operating system and application developers are a key target of hackers so that they can find and perhaps even install their own vulnerabilities.  Great care needs to be taken in the life cycle of critical software.
Microsoft never disclosed 2013 hack of secret vulnerability database

An interesting example of of a malware attack on medical devices for corporate espionage. Also highlights cyber-attacker-for-hire group called OrangeWorm.
Hackers Go After X-Ray, MRI Machines for Corporate Espionage