We live in a rural area, we found an adult native animal in a seriously injured condition, we were advised by our closest wild life rescue organisation that our only option was to have the police euthanise the animal as quickly as possible. We followed this advice. Is this halachically right
Yes, as there is no available treatment [or at least anyone willing to provide it] this will end the animal’s suffering and remove danger from humans.
The Chazon Ish is cited (in Dinim Ve-Hanhagos Mi-Maran Ha-Chazon Ish Vol. 2, p. 40) as having been asked by a member of the family how a dying fly should be treated. The Chazon Ish responded that the fly should be killed, so as to prevent its extended suffering.
While it is possible (Tzaar Baalei Chayim Chap. 4, no. 3) that this ruling will only apply to small insects, and not to larger animals, such as mice, and so on. The reason for this is that with regard to larger animals, killing the animal—even to prevent its suffering—may be perceived as an act of cruelty, and therefore it should be avoided even for noble motives. However, here where it will be done by others and the animal may pose a danger, this would be the proper thing to do.