It depends on whether he's bad and doesn't think he's bad or he's bad and he doesn't know he's bad. If it's the former, he could literally kill the startup. I

It depends on whether he's bad and doesn't think he's bad or he's bad and he doesn't know he's bad. If it's the former, he could literally kill the startup. If it's the latter, I can honestly say I have only met a few programmers who were incapable of learning good coding practices.