Generally in economic theory companies in perfectly competitive industries have zero "economic profit" which means they earn enough profit to compensate the owners for capital invested. Effectively the owners are getting fairly compensated for the risk of investing their capital.

In a monopoly the owner is getting compensated not based on risk of investment but rather how much can be extracted from the customers. This is clearly good for the owner and not so good for everyone else.

Creating a monopoly based on a superior product is a good thing for everyone since you are innovating and creating something that was not there before. You should be able to hold this monopoly if no one can effectively reproduce it. However, any kind of regulation to maintain your monopoly is likely bad for everyone except yourself. In theory, without regulation you would expect most monopolies to fall or have reduced power as other players come into the market.

I think Peter Thiel has a very good way of thinking about the motivation for innovation in a capitalistic market. However, outside of brand new innovations, monopolies are likely bad for a society as a whole.