Regardless of whether his viewpoints are valid or not, I think it comes down to whether or not that type of rhetoric and discussion is appropriate at work. No one is entitled to a job, especially a job at Google, and Google has a code of conduct that this guy violated. Companies have always had stricter rules about workplace behavior and communication than the public sphere has. This is not surprising, and should not be surprising to anyone who’s held a job. If someone at Google released an internal memo encouraging discussion and presenting “valid” reasons he thinks the Holocaust didn’t happen, that would be deemed similarly inappropriate, regardless of how “legitimate” his points were.
Google is having the discussion in spite of this guy being employed there. Firing him doesn’t necessarily stop the discussion, in fact the whole nation is talking about this issue now. There is a way to talk about Google’s diversity protocols, especially if they as a company say they “welcome” discussion on the topic, without making a hostile work environment. It is definitely difficult, but it is possible. This guy was not smart enough to figure out how to do it.
