Why do cops even need guns, if everybody else is already disarmed? LOL. (Wisconsin has no concealed carry law, I ubderstand.)
True, but that law doesn't affect long guns, nor possession of handguns. The people in general are hardly disarmed of effective weapons.
Even if they did have concealed carry, it probably wouldn't have helped. Almost all CHL laws restrict it to people no younger than 21, and all the victims, and the killer, were under 21 in this case.
You also have a point though, they were in effect disarmed, because no one takes their Mini-14 or "Serious Social Purposes" Rem 870 to a party, except maybe on an "out the vehicle" basis. If the party goers had any access to firearms, it would have had to have been those owned by the hosts.