First of all as demicav said well done for caring enough. Reptiles can often survive injuries that would kill and mammal, so killing it may not have been the best way forward anyway. The slowworm may not be dead as I often find them upsidedown for some reason
Its sad to hear yet another case of a cat killing a slowworm - in fact cats are probably the main cause of death to slowworms these days, and is probably the reason there less common in gardens these days
