Warning: Major, major Stranger Things spoilers ahead.
We mean it. They're huge.
It’s a real shame Billy becomes possessed by the oozing monster in Stranger Things season 3, but it’s not entirely his fault that it happened. The kid just wanted to hook up with one of the moms in town and next thing you know he's all sweaty and possesed. But there is a silver lining to Billy (Dacre Montgomery) going from emotional terrorist in season 2 to an actual monster in season 3: He somehow actually becomes more human as a result of the whole Flayed ordeal. He's still a villain for much of the season, don't get us wrong, but he also sees a bit of a redemption arc, starting with Eleven's (Millie Bobby Brown) visions of Billy as a child. Not only are these visions visually stunning, they also explain why Billy is the way he is.
When he's first is introduced in Season 2, we immediately know he’s a bad boy because he drives a fast car, smokes, wears tight jeans that drive the ladies wild, and wears one dangly earring. He’s also a bad boy in the literal sense, because he’s abusive. He yells, he throws punches, and he hates that step-sister Max (Sadie Sink) is spending time with the other kids, going so far as to try and attack them — as you likely recall, Billy is stopped when Steve (Joe Keery) intervenes.
But season 2 gives us a glimpse of why Billy is like this, making it clear that his anger issues stem from his father (who married Max’s mother, making Max Billy's step sister). At one point, Billy talks back to his dad, and the older man pushes him up against the wall and threatens him. Billy tries to put on a tough face, but a few tears roll down his cheek, showing us that he’s not as strong as we might think.
Now, we know even more of his heartbreaking backstory. After Billy is possessed by the new monster in season 3, Eleven tries to track him doing by entering her void to find him. Because he’s now part monster, too, Billy is able to sense Eleven is searching for him and manages to keep her at bay. That’s then when Eleven realizes she has to see where Billy’s been to find the actual source of the monster.
In the void, Eleven follows Billy’s memories and finds him as a young boy on the beach with his birth mom. He’s a happy kid here and wants to keep swimming in the water, but his mom is clearly anxious that Billy’s dad will be upset they’ve stayed out too long. Off in the distance, Eleven sees some rolling storm clouds and runs to them, and finds an older version of Billy in there.
This is where his anger issues start. Billy’s dad yells at him for not being strong or good enough to play baseball, and later, Billy witnesses a fight between his parents in which his dad strikes his mom. That leads into Billy talking on the phone to his mom, who has left him and his dad behind to start a new life somewhere else. That's right: She left without little Billy, which has to do a number on just about anyone — especially a boy who's grown up withstanding verbal abuse from his father.
In her vision, Eleven basically witnesses all the past trauma Billy has endured that made him so angry and violent, all of which made him a perfect candidate to be taken advantage of by the monster and forced to do its bidding. It's incredibly hard to watch, but essential not only to Eleven and co's mission, but to making Billy a full character.
This is where it get majorly spoilery.
Later, when Billy attacks Eleven and brings her to the monster as a sacrifice, she uses the few happy memories that he has — like being with his mom on the beach — to snap him out of his trance. It actually works — because as we now know, Billy is traumatized not evil — and he comes to his senses and realizes the havoc he’s wrought upon Hawkins. Instead of letting the monster take Eleven, he sacrifices himself and is killed in the process, redeeming himself for all the bad he’s recently done, and breaking his step-sister Max's heart in the process.
And while Billy is gone, the memory of his sacrifice will surely weigh on the kids going into Stranger Things season 4. While none of them were necessarily close with Billy, he was Max’s step-brother and she made it clear in season 3 that despite their troubles, she does care for him. At the end of the season 3, a flash-forward from three months after the July 4 showdown at Starcourt mall shows us that Max is still coping with Billy's death, and now she has her own share of first-hand trauma. While it's great that Max is becoming a fuller character, hopefully she's not looking at a season 4 path similar to Billy's.