This makes me sad. My interpretation of these events was that a toxic employee instigated a witch hunt and was successful. By simple observation of numbers here, you probably disagree with this. But for the few of you that do agree with this point of view, take it as a reminder of the crucial role of hiring. You can easily hire the person who ruins your company's culture and causes your downfall.
Regardless of the facts of the case and whatever damage it may have done to GitHub's culture, if this incident and this individual's departure are enough to cause GitHub's ruin and downfall, then that's evidence of an absurdly fragile company and culture.