In "East Island City" you'll find Little Jacob's friend Badman sitting on a bench. He'll also appear on your map as a person icon around about the time you pass Brucie's mission "No. 1". Walk up to him to start the side objective.
He wants to kill some of the Russian gangsters, so grab a car and drive him to the Russian mob location. Make your way down the alley way, using cover techniques to pick the mobsters off one by one. The last guy might try to run away, so chase him and murder him and Badman will give you thanks for assisting.