Nolan Sorrento is the bad guy. There are no shades of grey here. He's all bad.
Cline says he named Nolan after Nolan Bushnell, the founder of Atari and creator of Pong (source), not because Nolan Bushnell is a bad guy, just as a tribute. We're not sure how we'd feel about having a guy who would kill a whole group of people to gain control of a video game named after us, although Nolan Shmoopento has a nice ring to it…
Sorrento is the head of Innovative Online Industries, also known as IOI, the world's largest ISP. In the Hunt for Halliday's egg, they employ a vast army of people called the Sixers, because all of IOI's employees use their ID numbers as their avatar's name, and these numbers all start with the number 6. Why have they put so many resources into a videogame Easter egg? To gain control of OASIS. Why do they want control of OASIS? To charge people for it. Gasp!
According to Wade, if IOI finds the egg, then OASIS, would "become a corporate-run dystopia, an overpriced theme park for wealthy elitists" (2.32). In other words, it would become the real world, and since the OASIS is an escape from the real world, no one wants that.
We'd like to take a moment to say, do not invite Nolan Sorrento to any Easter egg hunts at your house. He'll probably kill your family in order to get to the Cadbury Cremes. (Okay, we might do the same thing. They're just that good.) Sorrento will do whatever it takes to claim Halliday's egg for IOI. Og says that IOI and Sorrento's "sole purpose is to exploit loopholes in the contest rules and subvert the intention of Jim's will" (12.32). We're not sure if this is the case since "Halliday's contest doesn't have any rules" (14.35), but we doubt that Halliday wanted a giant conglomeration taking control of his creation, and, you know, killing people while they're at it.
But here's the weird part: Nolan Sorrento and Wade Watts aren't that much different when you think about it. Both are in the contest for personal gain (after all, evil soulless corporations are people, too). And IOI's staff teaming up to find the egg isn't that much different from the gunter message boards sharing information about the same thing. One thing the clans don't do, which sets Sorrento apart from the rest and puts him on the pedestal of Extreme Bad Guy, is kill people. Real people in the real world.
Yep, dude's a murderer. He authorizes a bomb to be detonated in Wade's trailer park. He sends agents to throw Daito out a window. And he has surveillance teams on all the remaining High Five gunters—Wade, Aech, Art3mis, and Shoto—and is prepared to assassinate them whenever it's expedient. Without question, Sorrento is evil to the core. So the next time you complain about your Internet Service Provider, keep in mind: at least they're not trying to defenestrate you.