Calling somebody a "rat" qualifies as an insult no matter where you go, even if you're not an old-timey mobster. Try it at work tomorrow, you'll see. Nobody likes it.
That's because rats, in addition to being filthy disease machines, are the exact opposite of whatever makes humans good. Rats don't know how to cooperate or form societies -- they just stampede toward whatever rotting garbage they want to eat next, climbing over one another and probably shitting in each other's faces in their mindless, selfish scramble to eat and breed and do nothing else.
"It's actually pretty boss."
But one thing science has taught us over the centuries is that animal behavior that seems random or horrible on the surface takes on a whole new light if you watch it long enough. And rat watchers have seen them exhibit some pretty loyal, altruistic behaviors with each other. Ironically, finding this out required scientists to set up a cruel Saw-like chamber of psychological torture.
They took two rats that had shared some cage time together, placed one in a restraining enclosure, and left the other to gaze at his trapped friend, while the scientists presumably masturbated furiously. The captive rat expectedly started letting out wails of distress, spurring his pal to lose his shit and start trying to save him. In a clear show of empathy, the rat even ignored a pile of treats nearby, displaying that he wasn't after any reward -- he just wanted his friend back.
"Professor, the mice have reached Shatner-level self-awareness!"