I can think of a couple possible theories.
First is the possibility that they were struggling with their self-perception at the time of the game and may have lashed out because of that.
Second is the possibility that they were roleplaying a character radically different from their usual personality and they took it too far. Personally, I've played racist characters in the past, but I'm careful to let the others know that it's an act. This person could have forgot to do that.
Granted, there are other possibilities, but I'm trying to give this person the benefit of the doubt.
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"You can't take the skies from me"