Has anyone actually hired a real hitman with Bitcoin? In pretty much all cases that I've heard of, the hitman turned out to be either a cop or a scammer.
You make a good point that, blockchain or not, ultimately the problem is not technological in nature. Bitcoin facilitates drugs and hitmen as much as ability to mail someone a letter with bank account details attached (account with money). As usual, the solution is to make the activity not worth the risk by posing as either party.