Venom is a symbiote, but he's not attached to Eddie anymore... He's attached to Y/N stark, he even has his own form because of her but he likes being attached to her. Venom got his physical form because on the connection he and Y/N have, like a soulmate connection.

Y/N is pushed and used by the Avengers, her own father, Tony Stark uses her and pushes her away. They all call her names, basically neglect her. So venom convinces Y/N to be the bad guy, even though is Venom just asked, Y/N would