Yeah, better to let them finish killing the room and then themselves long before the cops show up to bag their body than them only killing a fraction of the room before taking a bullet from someone who is allowed to carry and is in the room or nearby.
Your fantasy is better because if america added the strict no gun laws of Europe all the criminals will turn in their guns and america will be gunless in no time. THREAT AVERTED. You can't even compare Europe to the US because there are so many existing firearms in the country that any law to restrict law abiding citizens from possessing weapons won't hinder the supply out there for the criminals by any substantial amount.
Where is the example of a country that had guns aplenty (about a gun for ever person in the population) that went to strict gun control? Those are the only stats that could help your argument...if they were favourable.