The cats were alive. As to why they did it, I guess the same reason people have done horribly evil stuff since time began. Part of it is the fact that inside we are all truly animals. Part of it is mob mentality which tends to suspend the logic and decent parts that are locked in side. And, part of it is probably brain washing. I guess it's the same reason why German soldiers listened to SS officers when they told them to stuff Jews in ovens and gas chambers and the same reason people fly planes into buildings at the command of a charismatic leader.