Is Venom a villain in the movie with Tom Hardy?

The term "villain" is relative because these are different storylines.

The villain in one story can be the protagonist in another story.

Venom, as a character in the Spider-Man franchise, is usually an antagonist. Since the protagonist (Spider-Man) is a hero, that usually means Venom falls into the "villain" category. Occasionally Venom and Spider-Man team up against other villains, but in films like Spider-Man 3 (2007) he's an antagonist.

The Venom (2018) movie takes place in its own universe and storyline. Spider-Man isn't in the story. In this movie, Eddie Brock is the protagonist. By extension, the Venom character is also a protagonist, and is not the villain of this movie. Venom isn't virtuous or conventionally heroic, so he could be called an "anti-hero" instead.

Speaking just from the movie's perspective, as that is what you asked, I'd say Venom is the protagonist, or good guy. Carlton Drake, and as an extension, Riot, is the antagonist, or bad guy, as his motives are clearly not wholesome.

I would say Venom is another of countless "Anti-Hero" characters we see now in the movies. TVTropes defines them as:

An Archetypal Character who is almost as common in modern fiction as the Ideal Hero, an antihero is a protagonist who has the opposite of most of the traditional attributes of a hero. They may be bewildered, ineffectual, deluded, or merely apathetic. More often an antihero is just an amoral misfit. While heroes are typically conventional, anti-heroes, depending on the circumstances, may be preconventional (in a "good" society), postconventional (if the government is "evil") or even unconventional.

Let's not forget, this guy eats people that commit petty crimes!

So, in his own movie, he fights worse villains but yea, I would see why Spiderman would stop him from devouring another criminal.

Like the Punisher in the comics, he doesn't always see eye to eye with Spidey or other traditional heroes.

On the behalf of story perspective related to Venom, it's described that it is an alien which can't outlive in atmosphere of earth without companion.

So, eventually Venom always searches for a person who's in search of power or influenced by it or even finds leverage. It takes control over that person and bounds with it. If it's successfully connected to that person meaning that now both can leverage upon each other.

In such a way, Venom offers Tom Hardy cure to his cancer and also a support from him to live in his body for survival of it's own. So it's like Tom is the driver and venom itself is a car.

So what you can consider venom is that it's a vigilante which is under influence of it's host.

If host is willing to act good guy meaning that Venom will also drive thought that scenario (In this movie), and if it's hosted by some bad or evil guys it'll be evil. (Why? Because they're psychologically connected !)

*See the Spiderman 3 for such reference when Eddie connected to symbiote feeling hatrated towards Spiderman and venom beacoms villan.

So Venom is neutral in such behaviour and totally depends upon it's host carrying it. It just finds leverage or weakness of host to make deal with.