In SA OC Siman 316 the machaber in sif 7 writes that if someone catches nachashim and akravim that are mazikim in order that they shouldn’t hurt them, is muter (if it’s for “refuah” then it’s ossur.)
Later in Sif 10 the machaber writes that shar mazikim (like a nachash and akrav) when they won’t kill you, if they are still running after you then it’s muter to kill them. If they are not running after you then it’s ossur. However the machaber continues then you may be darsam lifi tomo even if you are miscavan and may kill them that way.
My question is why is it that in Sif 7 the machaber says it’s muter to catch them since maybe they’ll hurt you. However killing them is only muter if they are running after you. What is the difference between killing and catching. In both instances this is a malacha shain tzricha l’gufu. True the machaber does say you may be darsam lifi tomo (when they are not running after you.) However why is it ossur to kill them b’yadaim in such an instance but catching is muter.
Answer:
The difference is apparently that with regard to trapping, the melachah is defined as catching an animal for the purpose of keeping and using it. In this case, where the catching is done for purposes of being rid of them, the Mishnah Berurah (316:27) considers it mis’asek, meaning that there is no melachah at all.
Killing insects/animals is different, in that the melachah is defined as killing, and killing for the purpose of removing is still considered killing.
Although the Mishnah Berurah begins that it is einu tzericha le-gufo, it appears, from the halachah of killing insects, that this is not sufficient for blanket permission to catch and to kill them.
The question remains a nice question.
