Having criminal contacts in both countries doesn't necessarily mean somebody trusts their criminal compatriots in both countries. Particularly when you're talking about a crime that's more severe (murder) than what they're normally trusting somebody with (smuggling perhaps.) I'd guess there are a lot of informants in organized crime. It seems the sort of business where the least trusting survive the longest.

If somebody took the risk to find a contract killer once and didn't get caught, it makes sense to me that they'd try to avoid taking the risk a second time by using the same killer.