Well, you are a gentle person and I will respect that.
With that in mind, here is a quote from the Humane Society:
"Lethal control
There are no truly humane ways to kill rodents, only methods that are less inhumane. Rats are killed with poisons, snap traps, glue boards, and maze-type traps...
Mix 1 part plaster of paris with 1 part peanut butter.
Add a dash of vegetable oil if needed for consistency. Do not use water.
Roll mixture into marble sized balls.
Place in an area accessible to rats, but not much else.
As the balls disappear, so will your rats...
Use a maintenance...