There really is no pretty way to prekill rodents. Building a little gas chamber is the best method...Co2 canister, tubing, and a rubbermaid container will do it. Cervical dislocation is considered a humane way to kill them, but if not done swiftly and with conviction, you're just going to bring a world of pain to the rodent. In your case, with what I'm assuming is just a single snake, live may be the way to go. Buying f/t in small quantities isn't necessarily a money saver, and buying in large quantities will leave you with a bunch of stale, freezer burned rodents.