To understand Thiel's position, you have to broaden your idea of harm past immediate harm to a single group of people.
While I'm sure Thiel has personal economic concerns, his stated primary moral concern is preventing wealth redistribution, socialism, and the mass-scale violence that those programs have caused in the past. You can choose to believe him as to whether this is his true motivation or not, but you asked for an explanation of this worldview and that's what he's stuck with over 10+ years now.
While you might be more worried about Nazis, Thiel is more worried about things like 1905 and 1917 Russian revolutions, the ensuing Red Terror, and mass starvation/poverty. About 20M people died in Russia during this time, mostly from starvation but up to 200,000 dead by being executed by the state for having the wrong political beliefs. And many more during the various violent uprisings and civil wars.
Some of you reading this are going to huff and just disregard it, but for Thiel, preventing potential massive tragedy in the future is likely worth a tradeoff of some people being "harmed" now.
Edit: the number of replies to your comment which basically amount to "he just doesn't care about others/he's rich so he's evil" indicate that even the Hacker News audience is not immune to believing in lazy explanations and supernatural forces. Google "The Myth of Pure Evil" for more on this.