That poor thing, reading this thread made me SO upset!
Try and feed it something - and why is there poison around? If you care so much about it dying peacefully, GET RID OF THE POISON! But I know it's not your poison, it's probably just all around the building for big rats and stuff. Personally, I would never be able to kill anything. Ending an animal's life just because "we can" is the worst. I would take it to the vet - honestly, but I'm crazy.
I hope it all turns out okay!
"If you are going to do something, do it well. And leave something witchy."
-Charlie Manson