A kitchen knife can work across intercontinental distances. Buy plane ticket to city where target lives, buy knife in local store, get taxi to location of target, stab target to death with knife. It's less glamorous than drones and missiles, but it's just as effective in the scenario that kicked off this thread, where one civilian decides to kill another even though he'll be writing off his own life in the process. Shinier weapons don't change that fundamental.

Drones, at the very least, would not be that hard. They're already developing a solar-powered drone that can stay in the air for years; crossing a few oceans are definitely in the realm of possibility.

http://www.popsci.com/article/technology/solar-powered-drone...

Drones have been launched intercontinentally, although with a significant failure rate, since last millennium: https://en.wikipedia.org/wiki/AAI_Corporation_Aerosonde

600 km/L, that's very impressive fuel economy.