What is the mindset of someone who says they will put poison out if your pet rat escapes?

They are inconsiderate and I really don't know what goes through their minds. You shouldn't say things like that about peoples pets even if you are not fond of that particular animal. 

On a related note, I don't like it when people use rat poison for pest rats either. Snap traps are best. Poison is a slow miserable death and if predators eat the poisoned rats they too become poisoned. 

A lot of people, and I hate to say it but especially women, are afraid of rats and even mice.  Plus they have heard stories of how they carry disease - which is pretty rare nowadays, since the government keeps tabs on such things.  Also of course, the wild ones will bite if you grab them and they can't get away.  They are not aggressive, but they sure are defensive!  I have kept pet store rats and mice, and also wild mice and rats that I have caught, or someone else caught for me.  And I have been bitten by some of the wild ones too, but they were healthy and it wasn't a big deal. You can tell I'm a rodent lover.  

I have also killed "pest" rats and mice in my house, because they are destructive.  

I have read though that New York rats can be different, there are millions of them and sometimes they can get into a big crowd of rats and attack - or fight back when attacked. But that's a special case because of the environment.  I'll bet the rat population went down due to restaurants being closed during the pandemic.  

My parents were the same way about pet rats. Rats have a bad reputation, particularly among older people. They were considered vermin and disease carriers many years ago and certainly not pets.