It is pretty simple really, he violated their standards for employee behavior. Regardless of that since California is an at-will employment State they could terminate his employment immediately without cause. As the court ruled the things he said and did were not Federally protected therefore Google's actions were perfectly legal. Indeed as a corporation, they would risk prosecution if they had not acted to protect Google shareholder's interests by firing him.

I'm sure Damore will find no shortage of brogrammer enclave startups in Silicon Valley to hire him and if he is really any good he could do contract work at $200+ an hour. In addition, Google probably did him a favor since I'm sure he has already made a small fortune peddling his tale to the shallow state media and alternative news, although I'm sure that was never their intent.