Is money the only possible motivation for a politician or politically active individual? Of course not. It can be the main motivation for those who aren’t rich yet, but once someone is rich they still have to be motivated to use the political means against their fellow humans.
Otherwise they wouldn’t.
To pretend that a billionaire only has your best interests in mind since he doesn’t need more money is to ignore all the other factors which could be motivating him. It is also ignoring the fact that being a billionaire doesn’t automatically satisfy the hunger for money.
He might want even more money.
He might want power.
He may have delusions of godhood.
He might actually want to make people suffer.
He might be insane.
He might honestly believe his ideas are good, but be frustrated that people don’t willingly comply, so he cheats and uses politics to force compliance with his idea that’s so great he has to force people to go along.
To imagine that the only explanation is that he’s a wonderful, caring person who only wants to ensure the flourishing of humanity is to ignore that he is using the political means instead of the economic means. That means, no matter what he is motivated by, or what good he believes he is doing, he’s carrying out his plans in the most evil way possible. Even if you like what he’s doing and support him.