This is another Comic-movie story arch that got left out, that probably shouldn't have been, because it will be a little bit more awkward to throw it in in the next 2 movies, but Brock is basically the Milton of the daily Bugle, he is constantly getting the short end of the stick, with his job, with his relationships, with his house, with his whole life. Yes, Peter is (more or less) responsible for Brock losing his job, but on top of that, Peter is constantly showing him up by producing better stories and photos than brock can.
So, Brock is ready to end it all, he goes to a church, and by coincidence, spider-man is there as well, currently infected by the venom symbiotic, Spider-man is there to ring the bell, which will release him from the Venom, As he rings it, the venom(represented as a goop) falls on Brock, Venom is an intelligent being, and it takes brocks malice and anger towards parker, and exploits it.
Carnage comes into play a little later, after brock is captured and jailed, the venom produces an offspring, that being carnage, which attaches itself to Brock's prison inmate, a deranged serial killer.