There are a lot of villains in Westworld; it is a western, after all. There are bandits and murderers who are fulfilling their narrative functions with their bad behavior, but there are also humans who exploit the androids (called "hosts") that populate the park. The technicians, creators, and visitors treat the hosts like giant dolls who exist purely for their amusement – which is, of course, what they ultimately are. Still, even amongst all the other morally gray characters, one figure stood out in the show's premiere episode as a truly chilling baddie: Ed Harris' the Man in Black. He's got a deeper game in mind when it comes to exploring Westworld, and he has no compunction about wreaking havoc everywhere he goes. Though he's introduced as a visitor, the character's original inspiration has people wondering if the Man in Black is a guest on Westworld – or if he's another host playing a role.
Based on the information presented in the first episode, the Man in Black appears to be a human. He cannot be harmed by the other hosts but he can harm them all he likes, which is in line with the behavior of other guests. Some of the hosts become aware of their reality and "survive" injuries because they realize they can't actually die, but they're still left with visibly blood wounds. When the Man in Black is shot at, no bullets hit him or leave a mark. The way he speaks is also more similar to the other guests than it is to the hosts. There's no Old West jargon or faux Western accents with him; he has no regard for upholding the reality of the world and has an extreme sense of entitlement, just like the other guests.
There are other signs in his dialogue, too. The Man in Black mentions that he has been coming to the park for thirty years, making him very familiar with the other characters and showing that he remembers past narratives, which the hosts do not. When he speaks to the hosts, he refers to them as "you" and not as "us." He also explicitly says that he paid a lot of money to come here and play the game.
In the original film upon which the show is based, the Man in Black character (also called the Gunslinger) was played by Yul Brynner and he was a robot gone wrong who went on a killing spree. This has led some viewers to wonder if there's a twist in store for Harris' Man in Black and he's actually a host who has become sentient and decided to get to the bottom of the world he was created for.
However, that twist would end up contradicting a lot of what's been shown onscreen – and what's been said offscreen, too. Series creator Jonathan Nolan referred to Harris' character as representative of "everything human in the park," describing him as something of the ultimate gamer gone wrong. Nolan said the Man in Black was a "human guest who has taken the fantasy to its utmost extreme. He wants to play the villain, he wants to be the bad guy – omnipotent, manipulative, and evil."
The Man in Black might be playing a role, but it's one he created for himself. He's a real person who has immersed himself so deeply in the events of the game that now it seems he's trying to take control of it entirely. In this world, the robots aren't the threat. The humans are. And the Man in Black is the biggest threat so far.