Advertisement Morbid subject, but it's good to discuss the difficult stuff. I agree that massive trauma would be the most "humane" method for euthanizing a tarantula. I know it might be difficult to bring yourself to do. It must be done with confidence or you risk more suffering for you both. The technique of dropping a 4x4 on it sounds the most accurate and swift. Do this in a paper bag in the garden and you can bury the remains. Well done for asking for advice.