Easiest way to kill rats is cervical dislocation. Some big rats its a real pita to do so those big ones, some people will swing them by the tail and hit their neck/head on the edge of a table or something to break their neck fast. Also another way is use dry ice in a small container with holes in it put inside a bigger container with the rat inside(so the rat cant touch the dry ice). The dry ice will release CO2 which will put the tat to sleep and kill it. You can also achieve the same thing with home made CO2 chambers using Tupperware and CO2 cartridges for paintball guns. Google it or Youtube it and you will find how to make them.