Spoilers ahead for Stranger Things 3. Billy may go from bad boy to actual monster in Stranger Things 3, but in the end, he becomes the hero by saving Eleven. Based on what the Mind Flayer does to him afterward, it doesn't seem like Billy will be in Stranger Things 4. Unlike Hopper, we saw him die onscreen in Max's arms. So unless the series decides to show Billy in a flashback or the creatures of the Upside Down have a way of resuscitating his dead boy, this may truly be the end for our mulleted antagonist.
As you'll recall, Billy and Max were the new kids in Hawkins for Stranger Things 2. While Max quickly joined the D&D gang, Billy was more of a loner, and acted out due to his volatile relationship with his dad. Sure, plenty of people lusted after Billy — just as they do again in Stranger Things 3 — but he was a rather isolated character, which is perhaps why the Mind Flayer targets him in the premiere episode of the third season.
Despite the Russians' best efforts, the Upside Down is still closed off due to Eleven. So rather than keep Billy in some sort of suspended Upside Down state like it did with Will or kill him like it did with Barb, the Mind Flayer weaponizes Billy. He becomes "Flayed" and helps to enlist other townspeople in Hawkins, including his fellow lifeguard Heather and her parents, for the Mind Flayer's army.
While it might seem like the real Billy died the night he was attacked by the Mind Flayer at Brimborn Steel Works, Eleven is eventually able to tap into his humanity. After Eleven and the gang confirm Billy has been compromised by the Mind Flayer, she enters the Void to find where he and the rest of the Flayed are hiding out. There, she sees a memory of a young Billy happily surfing in California while his mom watches. So when Billy later attacks her in the mall and is about to hand deliver her to the Mind Flayer, Eleven brings up this memory. This shakes Billy back into reality long enough for him to decide to save her and sacrifice himself.
Billy could easily be labeled as a villain— especially because actor Dacre Montgomery excels at being so creepy and terrifying. But his dying moments are that of a hero. And just before his last, dying breath, he apologizes to Max for all the terrible things he did while possessed by the Mind Flayer. So although it may have seemed unbelievable in Stranger Things 2, it's hard not to empathize, and even feel bad for, Billy in Stranger Things 3. Hell, if he comes back in Stranger Things 4, maybe you'll even root for him.
Of course, the show hasn't technically been renewed for a fourth season, so it's kind of too early to say which characters will be in it. But this time, the Mind Flayer seems to have been destroyed for good — and Billy along with him. If that's upsetting to you, take comfort in the silver lining that Hopper's dead body was never shown. So if the Russians somehow captured him, as the end credits scene suggests, at least one Hawkins hero could return.