Bob Newby meant well when he told Will to learn to face his fears and fight them. He couldn't have known that unlike his dreams, Will's visions of the Upside Down were very real. But what's done is done, and now the Shadow Monster and Will are sharing a brain space. Or, as the scientists in Hawkins Lab see it, he's infected by the Shadow Monster like a virus. But while some hope to use Will's connection to their advantage, just who is spying on who in Stranger Things exactly? Please be advised: This post contains spoilers for Stranger Things Season 2.
When Will first connects to the Shadow Monster in Episode 4, the boys see this as something that could work to the party's advantage in taking down this mysterious creature. Dustin refers to it as Will having "truesight," meaning he can see the things that are happening in the Upside Down (and in Hawkins) in a way that nobody else can. These visions can also tell them where the Shadow Monster is heading next.
Mike sees it as more of a spying operation, with Will basically knowing what the bad guys are thinking, and letting the rest of them know what its plans are.
If this were 2004 instead of 1984, Mike would probably compare Will's connection to Harry Potter's scar in Harry Potter and The Order of the Phoenix, and how it allowed him to hear what was happening with Voldemort after the evil wizard returned.
But that analogy isn't quite right. Let's start with the fact that Harry and Voldemort's connection might be two way, but Voldemort doesn't really catch on that he has this ability to basically dial up and see what the poor Fifth Year with a massive case of PTSD is doing at any given moment. Moreover, Harry doesn't want to know what Voldemort is doing, and every time he connects it's emotional meltdown all over again.
Meanwhile, Will seems perfectly happy letting the Shadow Monster into his body. Instead of getting in a hot tub that might help keep a level of separation between them (and Will keep himself to himself), he demands the house be kept freezing cold.
Will isn't drawing the tunnels because he wants to tell Mike and his mother what terrible things are happening under the ground here in Hawkins. He's doing it so someone like Chief Hopper will go out there and dig it up and fall in, so the Shadow Monster can kill him.
And this is where things go terribly wrong because Mike (and everyone else) really want Will to still be on their side, even as the evidence mounts that he's losing himself left and right. He doesn't remember Bob at all, and when asked who Joyce is, hesitates a long time before stating "you're my mother" in a cold and remote manner, not "Mom!" in a child's cry for his mama.
But even so, when Will says that there's something down there, and the scientists should go check it out, they go without question, without hesitation, and walk right into a trap. All this despite the fact that Will told them "You shouldn't have made it angry."
Will's not spying on the Shadow Monster for his friends and his mom. He's spying on the humans and their cruel stupid ways for the Shadow Monster that wants to kill them all. And if he knows where they are, he'll call the demodogs there to kill them.
There's only one solution: to drive the Shadow Monster from Will, and hope that when he leaves, there's still enough of Will inside that he can come back.