We brought the rats with us to this place, we created them. They were happy in the woods living out their little lives start to finish unknown to the world. They are still there to this day, self-sufficient social animals. People keep them as pets.
Nothing in nature is malicious. Even with destruction or danger there is no malice, cruelty, self-deceptive excuse. There is horrible desperate suffering, of an indifferent starving-in-the-falling-snow kind, but no natural creature will deliberately enhance it. It simply is.
With intelligence, though, comes evil. Cats have a helping, and dolphins, and for us with these big healthy brains it is one of our signature features. From dirt we created filth. For population in balance, we created slums. From food we created a bloated mess of carbohydrates and sweeteners. Far more than we need and usually unhealthy, handled carelessly. Sewers and dumpsters from rivers and woods, overflowing. And so, the rat was born. A black-sheep cousin for those little social animals who raise up their babies in the woods.
Unknowing, they got pulled through into the nightmare of a niche we'd created for ourselves. Nature abhors a vacuum so they became something new. But they didn't want to. We did it on purpose. They did not. It's not their fault. It was us.