If you hire a hit-man on the dark web and pay with bitcoins I don't think it leaves any paper trail. Same thing with buying illegal drugs, weapons, child pornography, or anything else.
Even though the U.S. dollar is a fiat currency, it still has value, until it's replaced as the world’s reserve currency, which might eventually happen, or if states start to secede, which isn't likely. Hyperinflation is possible, with some prediction dates come and gone already, and some saying in another 10-20 years. The most likely reason for it that I can think of is if the mint started cranking out several times more money than they are now.