First off, there are a lot of ways to bring someone to "the dark side". Maybe it's blackmail. Maybe it's ideology ("the greater good"). Maybe it's just pumping their ego. Or maybe it's money, but not that much, and extra money can be helpful. There is a long history of people spying against their country or hacking for a variety of reasons, even if they had a job and a steady paycheck. You can't just pay people and expect them to be 100% honest for the rest of their life.
Second, most (known) vulnerabilities are not backdoors. As any software developer knows, it's easy to make mistakes. This also goes for vulnerabilities. Even as a paid software developer, uou can definitely mess up a function (or method) and accidentally introduce an off-by-one vulnerability, or forget to properly validate inputs, or reuse a supposedly one-time cryptographic quantity.