As zdean mentions, document. Take that seriously. Many positions I've worked in the past had an explicit "probation period" of 3 months. However good your hiri

As zdean mentions, document. Take that seriously. Many positions I've worked in the past had an explicit "probation period" of 3 months. However good your hiring process, sometimes you just get a bad fit. Try really hard to hire well, but be prepared for when it doesn't work. I'm guessing you didn't specify a probation period. Something I've heard good things about is moving someone into another role rather than fire them. I understand if you don't have that option as a startup. Also, if this person is truly negative then you really don't want them in the company at all. But if the negativity comes from them recognizing their own poor performance then moving them into testing or something may get you a win. This seems tricky, and relies on excellent judgement on your part.