If you have a "monopoly" which is this case just means you're the market leader by a wide margin, but in theory this could mean near total control of a market but the reason you have this total control is because your customers choose to buy from you over your competitors simply because you're better and or cheaper then this is a good thing. Customers are getting what they want. And the market leader in this situation will have a hard time abusing their position because any they'll have competitors nipping at their heels if they slip up.

I would personally phrase it pedantically as not that the natural monopoly is good but that it exists because they are good. A fine distinction and a baked in assumption admittedly that if they were to no longer be good they would no longer dominate. Technically there are some other variables like if an extended stay on top would atrophy any competitors or not and the time scale operated upon.

On a century scale individual murderers aren't a huge concern to society because they are either dead or infirm by then.