So really he made his money from selling his company, not just from the game sales itself. And I would argue that he more or less got lucky more than he "earned" it, which I think he has said as much in interviews before.
I can't really speak to if he directly exploited labor, but I think we can pretty safely state that Microsoft has in fact done so repeatedly, and so indirectly at least, Notch benefited from that as well.
Now does that make him morally corrupt for taking that offer? Maybe. But I think any one of us would take the same offer if given the chance. But the reality of the situation is that getting rich from this kind of success is very slim, and even then the labor and effort involved is very much disproportionate to what others are earning for much more effort. And if he was taxed at a rate where is was no longer a billionaire, but just a millionaire, then his quality of life very likely won't change too much while many other people would benefit, assuming that tax money is actually going to public services, that is.
Every billionaire are where they are at by being at least somewhat lucky. In a lot of cases they are simply lucky enough to be born to the right family. Some have worked to get where they are, but its not just hard work or effort that got them there.
And I would argue that there is no ethical consumption under capitalism, and I would also argue that that is the case for just about any other societal system as well. After all, none of us can live without being a burden or hurting others at some point. That's life. Its also more or less the concept of "original sin" that Christians go on about. Its fine to acknowledge that and only by doing so can society at large takes steps to reduce systematic harm where we can.
That being said, billionaires, by having more capital, have more power and influence under capitalism, so it can be argued that they get a larger part of the blame for systematic issues, especially as many of them do utilize their power to maintain the status quo or push for more harmful systematic policies. And the ones that aren't actively pushing such policies are still benefiting from such policies. And they could donate their fortunes to charitable causes, but in my opinion that's not something that we should have to rely on them doing and does nothing to solve the systematic issues at play.
At the end of the day, it's its not as if its a black and white issue, but the statement that no one "earned" a billion dollars is largely true in the sense that if you work hard or put in the effort, you can make it. Even in Notch's case, if he didn't decide to sale to Microsoft, maybe he might still be a billionaire today, but would he have earned it himself? Its not like he was the only one working on the game even when he sold the company. I'm not sure what the compensation the others working at Mojang got, but if he continued to independently develop Minecraft, getting to 300 million sales requires significant development effort between porting the game to various platforms and ongoing content updates. If he ended up getting the majority of the payout, then he would have very likely did it at the disproportionately at the expense of other's effort.
A billion dollars is a lot of money. Like a lot of money. I don't necessary think its wrong to have the opinion that billionaires shouldn't exist. At least in the system we have today. Now, I'd say that its the system that's the problem, not necessary any individual billionaire, but if they get to wieild the power that comes with their fortune, then its fair to have more blame for it as well.