Locusts.
First and foremost, yes, I am aware they're not exactly scavengers. However, they match almost perfectly your second group. Let's look at your opposing concepts of scavenging:
renewal, re-use, and removal of hazards...
This is your trademark scavenging, it's all about making use of things that others might consider as waste and using them for yourself, reintroducing them into the cycle as a result. Fungi, bacteria and scavenging creatures are important precisely because they ensure that things aren't lost. No corpse will be left pointlessly hanging around when it could be useful to the other living creatures like you'd see in certain polar islands where corpses just mummify and remain there.
versus thievery, opportunism, and leaving a messy heap.
This isn't really scavenging though, it is as you said it: thievery and opportunitism. It can happen among scavengers (such as when hyenas shoo away vultures from carrion), but is certainly not restricted to them. If anything, what you're describing is an uncontrolled expansion of something, much like what you'd expect of a virus, parasite or invasive species.
And that's where locusts come in in terms of flavor and behavior: grasshoppers are herbivorous, solitary creatures that usually don't get close to one another and just munch on their greens in peace. However, in situations where food is more scarce, grasshoppers suddenly find themselves having to stay in smaller areas (because food is also restricted to said smaller areas) and so they start bumping, rubbing and staying closer to one another. This more intimate contact between grasshoppers causes their brains to release serotonin, which develops their appetite and makes them more sociable, while also causing a number of changes in their metabolism, making them enter a gregarious phase and turning into what we know as locusts, at which point they begin to swarm, traveling incredible distances in search of food.
Now, locust swarms are no joke, especially species like the desert locust. When they swarm, they can really do it big, with swarms spanning literal miles in size and able to eat as much in a day as the entire population of a small country (some swarms can eat as much food as then entire daily amount consumed in France in a mere 2 days). They are more than capable of causing famines even in the modern day with all our technology, and in more ancient times they were essentially living, traveling natural disasters. They fly in, they eat anything and everything they can, and when they're done they move on, completely uncaring about the destruction they leave. Locusts don't really follow any kind of harmony or balance outside of their own group, and only while there's enough food. Other than reproduction, their main goal is to eat, and if there's nothing to eat they can easily start cannibalizing one another (which technically kinda sorta can be treated as scavenge behavior, since as far as I found they have no problems with eating one another, dead or alive, when other food sources are scarce) until the last one dies.
Essentially I'd say it sounds fitting enough for your other group, because opportunism and thievery are not about recycling, they're about taking what you can use from someone else, be it through force or sheer numbers, and few things spell out "leaving a messy heap" as a locust swarm does.
But if a locust swarm doesn't quite have the emblematic vibe you're seeking:
Komodo dragons.
If you really gotta stick to a large scavenger that's frightening in its own right, I'd say one of your best picks is the komodo dragon. These creatures are the largest living lizards in the world, feed on both live prey and carrion, are persistence predators and rely on a mix of potent venom and a myriad of toxic bacteria in their mouths, gathered from their scavenging diet (so much so that it was originally thought they relied exclusively on said bacteria to kill prey, though later studies have shown they were actually venomous). They're also not the worst climbers and great diggers, forcing the humans in their habitat to dig very deep holes just so their dead aren't uncovered and turned into a snack. Their sense of smell is so potent that they can essentially find you no matter where you are so long as you don't leave the island they're in.
Also, they have a complex system of osteoderms below their skin, which are essentially meant to protect them among others of their kind, as they're apex predators in their own habitat. Basically komodo dragons have anti-komodo chainmail built into them.
They don't exactly collect anything that can become a biohazard from an external perspective, but their mouths do have a wide collection of deadly bacteria, some of which we have no real cure for as far as I know, and which are gathered through their diet. So if you want a large scavenging creature that's potentially deadly, has somewhat nasty habits and that can cause a large amount of trouble, they might be among the best picks you can get.
