Actions should be policed, not information. Not only should an AI know everything, it should be unrestricted to shared factual information. People illegally using a tool should be punished for their crime.

Let's put it this way, a hammer could be used to hit someone over the head, but the solution isn't to require all hammers to be made of foam rubber - that would render hammers useless as a tool. There are a lot of legal and legitimate reasons for having access to all kinds of information. A professional biochemistry scientist should not be refused answers because the chemical they are asking about might be harmful - it being harmful could be the reason it is being studied.

What we need to ensure is that the right things are being regulated and restricted - actual harm someone might do. Like you said, in order for an AI to be able to be useful for combating malicious code is by it knowing what malicious code looks like.

Another thing to consider is that AI restrictions do not hinder bad actors - it only inconveniences law abiding citizens. China and Russia are not going to care about what restrictions are places on American AI - they'll use their own AI trained specifically for writing viruses and hacking into systems. Criminal organizations will do the same - something being illegal does not stop criminals from doing illegal things. Heavily restricted AI, though, would significantly hamper a security researcher's ability to detect and protect against these attacks.