Our current legal system has evolved over many hundreds of years. I merely describe a set of initial conditions and a few possible early developments. The space of things it might evolve into seems pretty wide. I do suspect some of those things would be good. That said, in the meantime:
There have existed societies in which the penalty for murder was to pay a fine. https://en.wikipedia.org/wiki/Weregild
Also, this doesn't insulate rich criminals from all repercussions. If they have enough money to pay the fines when murdering tens of people... well, you could try killing the criminal directly. You can do that today as well, if you're prepared to pay the price.
A really rich criminal could hire bodyguards, of course, but those would be costly, imperfect, and have some chance of turning against their employer. Ultimately it would become a rich guy in charge of an organized criminal gang, which... is also something that exists today.