I really don't think most billionaires know professional killers. Where would they have met them? The idea that someone at JP Morgan introduces you to them when their company IPOs doesn't pass the smell test for me.
You need to get connections by ensuring the person recommending you a pro-killer has incentive to ensure you are not discovered.
“Everyone has a price” or something.
In the US Amazon pays their SDE1s around $100k/year,with $160k total compensation that is actually relocation bonus bundled with vested stock paid in 4 years.
Elsewhere, salaries are even lower.
So, definitely not entry level FANG engineer money.
On the other hand, if you're wealthy and already a crime figure, working with and having work for you people who know you have organized crime contacts and resources, then yes, you could find "fixers" and assistants who would do just what you describe, probably for less money too.
They'd already be part of the same world and comfortable with requests like these while also knowing that you're not just some schmuck rich guy who can't really retaliate against them if they squeal or fuck up. Your anger over a mistake would have more weight.
Even then however, many lower-rung people in organized crime are also quite stupid and reckless, and think here too of the many wealthy criminals who also wind up in court because they were ratted on by their now protected, cooperating witness underlings.