Warren Buffett seems like a pretty good guy who has dealt honestly in his life and is giving away a tremendous amount of his money. Of course, I don't know he's not a vindictive, sinister, exploitative bully deep in his heart, because I don't know him. Just like you don't know him. Or Bill Gates. Or any billionaires.
It's a nontrivial thing to give away a billion dollars (or at least to do so in a way that doesn't end up losing a tremendous amount of it to fraud).
Also, many billionaires became billionaires by starting companies, and they can't give away all or even most of their money without losing control of the companies they've created. Why should someone wanting to keep control of the company they've built while they're alive make them evil?
You seem to hold the rather common misconception that billionaires are just people with checking accounts that contain huge numbers, and they could give away their money at any time very easily but instead cackle about how rich they are. This is not accurate. Most of them are billionaires because they focused on business, and what you seem to be saying is that everyone who starts a successful business must, upon their paper wealth being more than a billion dollars, give up the thing that they've spent their life focusing on, sell everything they own, and become philanthropists. Otherwise they're evil. But what if they didn't sign up to be billionaires - they just started companies and really like running companies so they keep running companies?
But society has decided, for the most part, that not having (non-figurehead) royalty in the first place is better, for many reasons. For all the good that a good monarch might be able to do with benevolent leadership, we've decided it's even better for nobody to have that power. The negative impact of a bad monarch is not offset by the positive impact of a good monarch, and "good" monarchs legitimize the existence of the bad ones. Same with billionaires. To say that it's good to have billionaires that promise to give away 99% of their wealth at death is to say that it's good to have billionaires that accumulate billions during their lives and retain the personal choice about what to do with 99% of their wealth.
I wouldn't call Queen Elizabeth II a good queen or Emperor Naruhito a good emperor because all their power is in the hands of a democratic government. I would say that this is the minimum expectation for a monarch today; it's not to their credit that they've done so, and they haven't in any real sense even chosen to do so. Whether they're good or bad people is up to what they do beyond that. If we were to say that the norm for billionaires is they must give away 99% of their wealth upon death, I suppose I could think of Buffett much the same way I think of them, neither positively nor negatively. But even so, both of them were born into their roles in a way that Buffett wasn't, and Buffett, to my knowledge, isn't advocating for the 99% promise to be compulsory.
If, say, King Salman decided that he wanted to use his power and wealth for charitable causes upon his death (and he certainly has been contributing to charitable causes), but retains the monarchy and passes it to his son, I would certainly not start calling him a good king! If he says that he can't dissolve the monarchy immediately but comes up with steps to gradually wind it down by the time of his death, and tried to work with the other reigning non-figurehead monarchs to do the same thing, and questioned the legitimacy of their rule if they didn't agree, then I might consider calling him a good king.