It is impossible to have Venom without first featuring Spider-Man which is the main reason why Venom fails as a movie.
Venom, as a character, needs Spider-Man. According to The Amazing Spider-Man comics, Venom embodies the features of both a harsh newspaper reporter Eddie Brock and an alien symbiote who got a rejection from Spider-Man. They both loathed Peter Parker, and their reciprocal enmity sparked off the creation of the vindictive and bloodthirsty Venom.
Although, director Ruben Fleischer and his seven credited screenwriters thought up an alternative origin story for both Brock and the symbiote, cutting off Spider-Man from the general equation causes lots of narrative gaps where Venom is unable to display any gracefulness, neither stylistic nor narrative.
Why does not Spider-Man feature in Venom? It is hard to explain, but it comes down to the fact that while Sony has the rights to the character, it has lately lent him to Marvel Studios so that Peter Parker (played by Tom Holland) could play also Captain America (Chris Evans) and Iron Man (Robert Downey Jr.). A win-win situation for both Marvel and Sony since it enabled them to co-produce the winning Spider-Man: Homecoming. However, it also means that Sony needs to set its disconnected Spider-Man Universe in motion without Spider-Man, filling Venom (and perhaps other ensuing movies) with a sense of incompleteness.
The reason for it sounds the following way: As Venom cannot take the symbiote's real origin from the Marvel Secret Wars limited series, it must come up with a new entity. The Life Foundation which is a generic lab devoted to space research and financed by the wicked Carlton Drake, is interested in the exploration of the stars for reasons yet not clear. It can concern the solutions to our universe's ecological, medical, or societal woes possibly menacing us somewhere in the galaxies.
Within the town, Eddie Brock is a business-minded, hard-news detective who is credited with pursuing suits like Drake and protecting the powerless Every Man. Eddie has an unfailing girl, Anne (Michelle Williams), and a high-potential gig, but he opts for putting them both under threat when, by sheer coincidence, he is appointed to write a puff profile on Carlton Drake.
There is a lot of story taking place here, and this story is easily discarded since the symbiote is certain to appear and pull focus. And it is a significant point, as Venom -- not having lots of comic book issues to rely on -- has to run through narration and character development in a futile attempt to make us care about Eddie, about Anne, and about the wicked machinations of Carlton Drake. But, in fact, we do not really care.
Venom gets better as soon as the symbiote appears, but it does not suffice. Venom is an alien, a creature who gets absorbed in its host and can convey, almost Jekyll and Hyde style, with its carrier. According to comics, the symbiote united with Eddie because they both loathed Spider-Man. According to the movie, Venom unites with Eddie because... well, because the seven credited screenwriters decided they could not go without it, too. The symbiote also bonds with Anne, as well as with Carlton Drake, giving formation to menacing Riot. The puzzlement is how these three people can also happen to be perfect surrogate hosts for the murderous alien symbiote.
In terms of narration, Venom is in a messy condition. Eddie Brock is not motivated to chase after Carlton Drake, has no solid reason to spoil his relationship with Anne, and barely any clear situation where bonding with the symbiote is the right thing to do. He does not see eye to eye with Venom. There is no reasoning in their cooperation. It is just understandable and acceptable, but not explained.
The entire movie is a pure disappointment, especially with this tease for a sequel that is unlikely to come out. This should have been a stepping stone for stories in this Spider-Man universe that could keep the lights on in parts of the wallcrawler's world until Marvel and Sony came up with reliable ways that would enable Peter Parker to get back. If Venom worked, stories zeroed in on Morbius the Living Vampire, Silver Sable, Black Cat and Kraven the Hunter could develop, complementing this universe and displaying that Spider-Man would be a desirable, but not necessary, inclusion.
What is worse, Venom made me believe in what I first was petrified of. It is senseless to make up stories around Spider-Man characters if there is no room for Spider-Man in them.