It's good that his actions were constructive, but they very easily could have been destructive (e.g. had he held slightly different values, had malicious advisors). That inspires worry in me.
We'll ignore the saving the whales thing, because it was purely an incidental side effect.
Benevolent dictators are bad, even if they did good, because them existing means a bad one could just as easily appear.
But I disagree that he could have done anything bad. What's the worst thing a monopolist can do? Charge unfairly large prices. By the time his the government got around to breaking up his "empire" for the politicians' own venal reasons, the global market had grown so large, that he already couldn't service it all himself. If he had raised prices as his end-game, the forces necessary to capitalize on that opportunity were already deployed and ready to strike at his market share.
The only dictator I see in the picture are the politicians/regulators who broke it up for their own political gain. Those are the dangerous ones because no matter what they do, we can't shop elsewhere.
It's not so much about Rockefeller being a monopolist as it is about him being a private citizen with truly enormous economic power, while being zealously devoted to a simple cause, and having the ability to rationalize away all doubts. These are worrying personality traits that facilitate doing great damage.
In the case of Rockefeller, these personality traits led to constructive outcomes, but the point I was trying to make toward the end was that it seemed a chance event that he would use his power for the philanthropic projects that he chose.
Here's a hypothetical: what if, for instance, Rockefeller had supported eugenics rather than medical science? How would that have turned out?
We can't know, of course, but it's an interesting question. I'll have to think about it and do some research! :-)