You didn't make an argument for why he's a bad character. All you did was list facets of the character.
As for listing facets of why he could be a good character...? Sure, I'm game.
He's got a cool looking design. Thematically, he's as polarizing to the concept of Spider-Man as you can possibly be. Whereas Spider-Man represents great power and great responsibility, Carnage represents great power with neither responsibility nor reason. The question then is, what would be the responsible way to deal with Carnage? He seems utterly alien, but there's still something wicked and human on the inside. As you mentioned, whenever he gets free (and he proves that he'll get free again and again), he'll immediately set in to butchering people. And that's not just the suit talking. It's Cletus feeling the same way. So what's the responsible way to deal with that? Is it to keep locking the guy up, knowing he'll always get free, or is it something else? Just because Carnage has been portrayed badly in the past doesn't mean he can't be a good character. Most villains were pretty goofy and one-sided for most of their existence.