Security thinking. If you want to keep your systems secure, you have to model intruders quite well, such that if you look at a device or environment you immediately notice how a malefactor could do things you don't want. You can do this and be the good guy.

Poacher turned gamekeeper works too. I worked at an antivirus company a while ago. Many of the researchers had been virus writers and l33t h@xx0rs in what they now regarded as their idiot youth, but had spent many more years using hacker thinking to defend against the people still doing that, and greatly enjoyed it.