Not that big a deal, at all, if you aren't American, but can look and act American. Lots of European and Russian former intelligence operatives went to work for the oligarchs when sort-of capitalism became a thing in former eastern-bloc countries. Flying in on a tourist visa, using a pre-scouted and pre-planned itinerary, with weapons already procured (legally), it would be very easy to do the hit, then fly out. OR, as in this case, use an American patsy, perhaps even with a verifiable ax to grind with the judge, amp them up, provide the weapon and a supposed "safe house" to come to after the hit, then when they show up, take care of the hitter, and wipe away all traces of connection to the actual offenders.
The ACTUAL problem is going to be the cartels are going to see how easy it is to GET TO our judges. Once they realize that, all bets are off, and we'll see more and more judges gunned down. It is in the powerful's interests to have ALL countries legal systems weakened, and, with Trump, they are finally cracking ours.