Let's look at this from the standpoint of energy - as in kCal of energy that can be created or derived from existing sources.
Autotrophic organisms get a free ride - energy from the sun is converted into stored energy in the organism. Heterotrophic organisms consume energy that was at some point created by autotrophic organisms. We arbitrarily cubbyhole different kinds of heterotrophs into different categories. 'arbitrary cubbyhole' means a definition to help us understand. The definitions are NOT all mutually exclusive. These terms arose way back when we did not fully understand what went on. So, this does not always apply to all living things. Bacteria can fit into many of the cubbyholes. Sometimes fully sometimes partially.
Staphylococcus aureus can thrive in your gut, until they produce enough toxin to kill you. Are they a predator or a parasite? If the bacteria do not kill you, they are passed out with feces. Human fecal material is mostly bacteria and fungi, with some complex carbs that resist digestion. Like bubble gum. Many of the endemic organisms, like our friend, S. aureus, cannot live long periods outside the body. Many die, some form spores. They lay dormant. What are they now? Still part of the feces or a dead body (if they killed the host). They may contribute fixed carbon when they degrade further and feces becomes home to new organisms. More energy is consumed until very little remains. The chemical nutrients and remaining fixed carbon can be utilized by autotrophs and their symbionts. Back to square one.
What is my point? You are trying to fit a square peg in an old-fashioned square hole. You have to determine how your prof sees these terms and parrot that back.
The terms are meant to explain both process and niche, with strong connotations (implied meanings derived situationally) . So when you talk about autotrophs and heterotrophs you are taking a wider perspective than a predator-prey-scavenger-decomposer model. Slightly different concepts to explain the same stuff.
You are trying to use exclusionary logic - in this example how can bacteria be predators? They can kill and even partially consume the remains.
You cannot define it that way. Answer is: they cannot be predators , because predator has a different connotation. Bacteria (or viruses) do not actively seek out prey, for example. Bacteriophages act as 'predators' in some offbeat sense, but we do not use the word predator that way. Because predator became a concept long before we understood bacteriophages. And so we correctly apply the word predator mostly to living animals we can see easily, or with a little magnification. We cannot see bacteriophages easily. They are viruses. And they are generally not considered to be 'living'.