> Would it be smarter as a murderer-for-hire to ask to be paid in a less-expensive (for now) but growing currency such as Etherium or Litecoin?

No, because the efficient market hypothesis.

If you know that some asset is definitely going to significantly appreciate in value why don't you just buy it and skip out the hitman thing? The answer is that almost noone has any better idea of what the correct price should be than whatever the current price is.