*There are sites on the darknet that exist to
hire contract killers for bitcoin scam would-be clients
FTFY

Most (if not all) of those sites are fake, you'll find plenty of articles on the subject matter. IIRC there's only one murder that's actually linked to a hitman that was hired via a hidden service and that one was an amateur that got caught pretty quickly.
And if I remember correctly there was another murder when the guy ordering the killing was so pissed off he got scammed he decided to do the killing himself and did not even denied it in court.
I doubt anyone would use the darknet to offer his services for real, this is not Uber to get better ratings, scamming gullible morons is far easier than risking your life.
That aside, I don't doubt that cryptos can be used for such payments, they can break the payment chain, the hitman avoid contact with the guy ordering by not taking cash, there are no bank transfers any anything, and what's more important, it's damn hard to prove you have actually paid for that and that he received that money from you. But the numbers of contract killers hired this way would be insignificant and not though the darkweb.
I just hate to read anything negative about Bitcoin.
And you think that sticking your head in the sand is the solution?
Bitcoin can be used for good things and bad things, just like eveything else, ignoring won't make them go away.