do you think a gazillionaire would be something to avoid?
No it isnt a good thing, because someone owning a fictional amount of money would make no sense and I dont like things not making sense.
in all seriousness though, in and of itself, no. But not all super-wealthy are good just as not all are bad. People who have wealth may generate it (provide services we want) or may get it through exploitation (literally stealing or inciting wars). So long as the wealthy person is one who largely made their money generating that wealth then yes its a good thing no matter how much they have (because it didnt reduce the amount anyone else had). However if they got that wealth through the destruction of wealth (for example literally robbing a bank) then no it isnt a good thing.
do you think they might be able to use that money to garner power in a way that even the smart democracies might have a problem dealing with if his interests went against those of the public?
That depends largely on the society. While I am opposed to just taking a persons money simply for having money I am all for making sure money can not buy influence in a society when it comes to our policies and governance. A persons money only has as much power as we let it as a society. If we all had integrity then that money couldnt buy anyone or anything that wasnt something we as a society are ok with. So if you find things are being bought with money that money shouldnt be able to buy (such as votes for politicians) then that is a problem.
Remember people can pool their resources into a common organization. So I dont care any more or less about 1 super rich evil dude than I care about 100,000 evil middle class dudes.
maybe buying all our telecommunications infrastructure and politicians and military industries?
They would be unable to do this if those non-billionairs simply refused to sell to them. Or if the government prevented it through anti-trust laws (I am against monopolies but all for the rich).