Lars was there mostly because he was bored, and a bored Lars needed out of the house or things got rearranged in odd ways (such as the sofa in the hallway of their building). That & free food. Socialization. Conversation! (as Darke had been quieter than his typical quiet self the past few days). So at the mention of free food, he had grabbed Darke, Lamont and Aimee (who had a night off!) and dragged them here.
He left Darke with instructions to not draw on the walls and went off in search of food and (probably someone to harass along the way).