The worst that will happen: Blindsiding
A narcissist has a weak spot; they think they are better than they actually are; when being the best is extremely unlikely, statistically speaking, particularly for a one-shot wonder: That is what your character is, he invented one thing and he was done. Inventing a trillion dollar tech idea does not make you a genius in anything BUT that tiny area of tech idea.
Even if, by some miracle, you are the best coder in the world when in a room alone (and what are the chances such a person produces one and only one hit?), you will not be the best thief, best assassin, best bomber, best con-man, best at corrupting politicians, best at white collar crime, or best shady lawyer, or best blackmailer, or best identity thief. Or the best at planting evidence to frame other people for crimes. It doesn't even make you the best hacker; your skill set is on the positive services side of the aisle that everybody loves, straights and crooks alike, or you wouldn't be so wildly popular.
You might think you can hire those, but you haven't spent a lifetime practicing the criminal crafts, you will just get conned. The people that have spent a lifetime practicing their criminal crafts, to occupy the real-life top tier in the world, have also had to develop the skills to lie convincingly and evade detection and jail.
Neither lying, or detecting lying, is in your wheelhouse; you have not spent every day of thirty years telling lies and worrying about being conned yourself: You have been in your head, planted in front of a keyboard, solving bugs and inventing interfaces for 12 hours a day. Not hustling drugs.
You have here a lack of experience, and the hubris of the bona fide genius: They think because they are exceptional in one area, they must be pretty good in others, and overestimate their ability in these other areas, which means they fail. After a few decades of wild success in one narrow field, ego gets the best of them: They branch out into politics, or company management. They have the money to be in charge, but turn out to not be geniuses in this new field, but novices: They didn't spend 20 years doing that, they spent it behind a keyboard, alone, trying to make a machine work.
Further, there is the attention factor: To a criminal, fabulous wealth is a magnet; the bigger it is the more it will pull the world's best to try and take it, exploit it, or get into your inner circle to influence you and it. Not the most brutal or most obvious criminals, but the most likeable that have done the best job at concealing their dark side. Are you the best at detecting their deceptions? No. They will infiltrate you at every level, often recognizing each other and colluding to take as much as possible.
As requested by the OP, the rich inventor does not turn evil: But his wealth ends up being used for evil, relentlessly, and he ends up conned out of it.