Billy likes horror movies because when he gets scared (he vehemently tells Steve otherwise) he screams. He screams, and cries a little, even. He loves it because he can express his emotions easier this way. He can't always be open, and sometimes he just wants to cry but it's like he's all cried out. Plus crying triggers his old mindset, and brings him back to that house. To Neil.
Billy likes to yell at the screen and throw popcorn whenever the movie character does something stupid, or when the killer is right behind them. He and Max argue about who's going to die next. He teaches El about the "fundamentals" of a horror flick, because underneath all the leather and curls, he's a good teacher and has patience (only for certain people).
Of course it becomes a ritual that Eddie breaks into their house in the middle of a scary scene. He jumps on Billy's lap, stretching out onto Steve's, using the same line over and over: "Hello, big boys. What're we watching?"
Steve has tried to throw Eddie out Uncle Phil style, but he just keeps coming back. Billy finds a kinship in Eddie, because he too has made it his life's purpose to annoy Steve Harrington.