Originally, Eddie Brock Venom was a Spider-Man villain, specifically a semi-nemesis to Peter who had powers similar to his, if slightly amplified, from the black symbiotic suit. For roughly 3 years from his introduction, his sole goal was the destruction of Spider-Man, by any means necessary. He was locked up as being "Criminally insane" for a few years. By the early 90s (in comic) though, Brock started being more of an "anti-hero", using his powers to stop minor evil doers and help folks surreptitiously (while also eating an occasional brain of a crook here and there, to satiate the symbiote's killer instincts.)
At some point though, Brock became a full-fledged hero, both doing a lot of good on his own, toning down the killing, and even working with other heroes like Dark Hawk, Deathlok, and even Spider-Man himself. The first instance I really recall of him being out-and-out "heroic" was the Maximum Carnage story line in 1993.... but even then, he was still mauling people, using excessive force, somewhat antagonistic to Spidey and it was a really special circumstance; Brock himself said he felt partly responsible for all the... well, carnage... that his offspring symbiote was causing. So Peter recruiting him was partly an act of desperation, partly a "no choice" situation, and partly a "My mess, my clean up" kind of thing.
Is there a specific issue (either of a Spider-Man comic, or his own series) or inciting incident (like the Uncle Ben killing for Peter) which inspired him to be a hero?
I'm speaking here specifically of the comic book, Earth 616 Venom, not the cartoon or film versions, ones from other universes with different origins, or any of the Agent Venom characters. This also does not apply to Eddie Brock's white "Anti-Venom" form.