The thing is that this guy clearly thinks some things that affect his ability to do his job. Would you trust him to write a peer review for a female colleague or mentor a black intern?

I think what you say is true about people searching for ways to express themselves when the lines between work and personal are blurred but fundamentally people have to attempt to be unbiased. It's pretty clear that this guy wasn't willing (or able) to do this and keeping him on would open up Google to a lot of bad PR and potentially lawsuits from people who's reviews/interviews etc that he was involved in.