I don't really have a problem with millionaires. Lots of people have a few million in assets, mostly due to house price inflation, but also famous sports stars, pop music, actors, etc., as well as entreprenneurs, and of course gold plated pension schemes were the underlying "value" can quite easily top a million for higher earners such as doctors and dentists. Millionaires can (and often do) lead pretty normal lives. (Yes, better than a cleaner, but just "better than average" houses, cars, etc).
What I DO have a problem with are BILLionaires. Just no need! No one "needs" to own their own island or private aircraft, super yachts, etc.
Trouble is the billionaires are movable and will just move to a different country/tax haven if they don't want to pay as much tax. Look at Lewis Hamilton buying (I think) a helicopter via trusts and the Isle of Man to avoid paying UK VAT! Just wrong on so many levels! But how can anyone stop it? He'd just buy his helicopter somewhere else if the tax haven status of Isle of Man was scrapped. Likewise people such as Tyson Fury rumoured to be relocating to Isle of Man (and it's not because he likes steam trains!!). Or Shania Twain who years ago "moved" to Switzerland for "creative reasons", i.e. creatively avoiding tax on her millions had she remained living in USA/Canada!
Same applies really to those "worth" hundreds of millions who I'd put in the same category as billionaires. I don't really have much of a problem with people with millions or tens of millions as lots of relatively "normal" people will fall within that kind of range.
I also can't believe that a lot of the billionaires have achieved it through legitimate/legal means, a lot of it must be drugs, weapons, money laundering, fraud, etc.
I'd love some kind of Worldwide agreement to ban tax havens or stop the mega rich billionaires paying tiny amounts of tax (in proportion to their wealth), but can't see any way of making it happen. The tax haven countries/islands rely on being tax havens so you'd have to pay/bribe those countries huge amounts of money to change their rules to prevent them being tax havens to compensate them for all the multi millionaires and billionaires who'd leave when there was no benefit of staying in tax havens! Never happen!