I agree that Carnage (like almost any character) has the potential to be written well and expanded beyond his original purpose. GG and Doc Ock evolved and their backstories and psychologies were fleshed out much later in development. But the thing is--Carnage was not. The character is the sum of what they are and Carnage has never moved beyond the killing machine he was created to be. There has been no depth dug into to find a likable or at least interesting (because how many serial killers are likable?) character. He has been one-note so there is is no incentive to say he is a good character. Like a person, he is the sum of his actions. The writers could have done more, but they chose not to.
However, the character's entire conception is his problem. He was created as the epitome of what was wrong at Marvel in the 1990s and comics in general. He was Marvel's reaction to McFarlen's Image Comics and why it was called the "Image Age." It was all style and pretty visuals meant to be mass marketed and sold, double sold and triple sold in 15 variant covers at a time. It was all about images and flash in the pan.
Carnage was created as a vehicle for Marvel to use in their flagship at this time. Unlike GG, Doc Ock, Lizard, etc. he was written as a tunnel vision character. And unlike say Kraven, who was brilliantly re-invented into an A-lister in the 1980s with Kraven's Last Hunt, he had no goal as he was created for literally his namesake. Bloody and violent carnage as violence (and sex) is what sold these comics, then. His purpose was to cut people up and be ultra-violent and there was nothing meant for him to do beyond that. Future writers could attempt to give him motivation or rationalization for these insane butcherings (they fell on the old creativity crutch of a bad childhood). Other than that no more has been attempted. He was meant to be eye candy and nothing more and that is all he has ever amounted to.
Also, there is the fact that the character is a shameless rip-off of Joker and excuse to ruin another character who also began on shaky terms, Venom. Venom had a weak origin but room to expand, however with him appearing every other month in Amazing Spider-Man Michillinie became increasingly lazy and started writing him as a one-trick pony. Well instead of expanding him (as Michillinie apparently had plans to kill off Brock and let the symbiote become a free agent), Marvel wanted to milk him some more and turned him into an anti-hero, the Lethal Protector. And worst yet, had him and Spidey say "Let all the murders and death threats on me and my loved ones be bygones."
Well if Venom was the "evil Spidey" who looked really cool...now we're going to need a new one who can be both Spidey and Venom's arch-nemesis. Enter Carnage. A cheap imitation of the Joker without the complexity, ambiguity or menacing mystique of that character. It is the agonizing question of who the Joker is and what he is going to do next and the creativity of his havoc and chaos that makes him the greatest villain in comics. We get a boring origin, name and background on Carnage, aka Kassidy, on day 1. Then you know what he's going to do as soon as he's out. He's going to kill the first person he sees. Whenever he escapes he must first murder his guards or latest warden.
"Kassady, what are you going to do next?"
"Kill you and the next person I see!"
What a compelling villain.
He was created to just be an excuse for another Venom-esque villain now that he was turned into a hero and so there could be blood running through the streets of of Spidey comics. In fact by the end of Maximum Carnage one really has to wonder why Spidey and the Avengers just don't kill this guy, even if it is against their code. Or what about the government? You'd think they'd want to get rid of him the first chance they get (and something tells me he wouldn't get much sympathy from a trial of his peers after killing thousands of New Yorkers). This guy is a huge threat who makes all the heroes look incompetent. But that is done to show you how "cool" he is.
I guess what could describe why Carnage exists and why some fans backlashed against him can be best seen in his original appearances. He appeared in three consecutive issues each one entitled in bold bloody letters on the cover "Carnage" with each subsequent issue adding "A villain so cool we had to say it twice!" or something to that effect. They were shoving this marketing toy down our throats.
Now, I personally like the concept of Venom and liked the character in TAS (and in SM3!
), but think he has been a waste of space in all comic book forms. But the potential is there. Carnage was created with no apparent room for that potential. A good writer could find it and use it, but to this day after nearly 20 years of existence, Carnage still has not gotten it and is likely not to at this point. For that reason I can say what needs to be said: Carnage sucks.