At this point, Killua likes Marui and finds him really amusing. He thinks he guy should calm down a little about that safety thing but otherwise he see him as an alright person. It helps that Marui gave him sweets. One of his few weaknesses

He's is a little and I mean little touch about concern Marui shown for him. Though he's not sure how long that's going to last once the guy find out about his family.