I was actually a big fan of Venom during the "lethal protector" era so that period I am familiar with. I "know" about the "truce" that Venom and Spider-Man made at the time. He moved to San Francisco and fell in with some group of homeless people who lived underground. He eventually moved back to NY and even became a government agent. By the late 90's, his prime was over so Marvel cycled him back into being a villain. Assigned to spook J.J. after he criticized some government agency or something, Venom tried to kill him and fought Spider-Man again. A bomb had been placed in Venom's chest to leash him, and when it exploded it drove him back to being a baddie. While he was still rarely an ally of other villains - he once joined the Sinister Six, only to betray them and try to kill several of them one by one - he was once again a foe of Spider-Man.
Some of those "setbacks" have still lasted many years, and mire the attempt to make him an anti-hero to a degree. By that logic, the 6 years Wolverine lacked his adamantium was just a hiccup.
After the start of the Joe Q era, though, a lot of wonky things happened. The cancer, the symbiote thing getting weirder, etc.
The fact that Vance Astro/Marvel Boy was convicted of manslaughter for killing his abusive father (who was trying to beat on Vance after Vance returned home from a battle, and was injured pretty bad from that) is only half the story. A lot of people forget how exactly the prosecutor proved her case. She drew a loaded gun in open court and aimed it at Vance. When he disarmed her with his powers without harming her, she claimed that proved he'd wanted to hurt his father, since he could have stopped him in a similar way. Even given a world with super-powers, one would imagined that if that REALLY happened, the case would have either ended in a mistrial or been dismissed.
Thus, I have come to the conclusion that every jury in Marvel's NYC just really likes convicting super-heroes when they get the chance. Villains get endless chances. That's why they keep getting out.
Eddie Brock's original vendetta against Spider-Man was always a bit deranged, and that was intentional. He didn't much care about what happened to the fake Sin-Eater; he only cared about the effect Spider-Man undoing his story had on his life. It is not uncommon for most people to blame all the problems in their lives on everyone but themselves. Many a Batman villain, either in the 90's animated series or the comics, came of some ordinary person's life being destroyed by someone else or a mundane thing. In ASM #300, the impression I always got was Venom was something of a stalker, since he collected clippings of Spider-Man, followed him around, spooked his wife and toyed with him before launching his attack (Venom, using his ability to block the spider-sense, grabbed Spidey's leg when he climbed a wall once, and then tried to push Parker in front of a subway train before outright appearing to scare MJ in their apartment and then fight him). And many stalkers seem to be unhinged or obsessed with their target. As an evil version of Spider-Man, Venom was a modern take of an old cliche; but as a stalker-villain, he was perhaps ahead of his time.
So in a way, Brock still being a bit obsessed with Spider-Man works.
The problem with Eddie Brock/Venom was that he became very popular very quickly, so a lot of stories were written about him in a short time-frame with editorial struggling to maximize his potential. This was the late 80's, recall - when Marvel was just starting to learn how to spam popular characters once they were hot, like Punisher and Wolverine. Many of his original stories were actually written by the same writer, but Brock's character sometimes jumped all over the place. Then other writers started writing him and as with today, things didn't always mesh and got complicated. The dilemma is that a character who was clearly intended to be a villain had to do a 180 so he could star in his own series as a vigilante - an edgy, lethal vigilante.
In some back-stories, Brock's father was vindictive and abusive, which didn't set him up well in life. I think there was some issues with his sister, too. In ASM #300, Brock notes how the deaths of innocents are always unpleasant - as he kills a cop who stumbled upon his hiding place in a church. Eventually that would spiral into Venom's quest to defend "the innocent" as a vigilante. The road to redemption for Venom came with Carnage, who was seen as being crazier and more dangerous. While Venom was willing to kill the odd cop or prison guard to escape or remain free, his vendetta was usually focused on Spider-Man (although he once pummeled Black Cat something awful - smashed her face-first into a wall if memory serves). Carnage, on the other hand, was a serial killer who killed random people for fun. Of course, had Venom really been vigilante material he could have always tried to act as one while still trying to kill Spider-Man, but, again, this stuff was made up as they went along, even if it contradicted other stuff.
When I was younger, I liked Venom for the design and because the angle of him being misunderstood as a vigilante appealed to me - I was in junior high, whattayawant? I saw him as someone trying to make up for an ugly past and trying to do good, but many people still saw him as a monster, and rarely trusted him. It didn't help that he looked like a monster, and usually killed criminals (or the occasional person by accident like in the aforementioned VENOM: THE MADNESS). At least he always had a twisted sense of humor. He was always supposed to be deranged, though - and that budded before the alien got to him. It just made it worse. The cancer gets blamed for some of that too, although that clashes with him being a weight-lifter before bonding with it, which justified why Brock was stronger than Spidey as Venom.
I heard the REAL original idea was to make Venom a woman - to address the lack of them in Spidey's rogues gallery at the time (especially since Black Cat had mostly reformed by then). But that changed. Spidey still has very few female adversaries. Commanda from UNTOLD TALES OF SPIDER-MAN would have been a good way to undo that, but she's been abandoned. This new Wraith is his closest in a while, at least since Paper Doll I think.
The symbiote was always a sort of dues ex machina in a way, and I don't think just having it be a monster would have worked. A lot of people thought Eddie's motivation was odd and out of left field, since he wasn't built up in the cast. Virtually every other revision of him in Ultimate or other media sought to correct that. USM made Brock an old childhood pal. The 90's cartoon built up Brock over several episodes as a rival photographer (akin to Lance Bannon at the time). The third film utilized that angle, while "SPECTACULAR SPIDER-MAN" was closer to Ultimate.