They mentioned arms because guns weren't the only weapons. Partisan's and pikes may have still been seen on the battlefield. The sabre was widespread especially among cavalry which brings me to the point of mass killing.
A cavalry man had a horse which could carry a little more than the average foot soldier. They could be armed with at least two pistols and than a sabre. Usually cavalry were used at that time when an enemy force was either fleeing or wavering on the point of breaking. So a cavalry man would be able to discharge at least two pistols and than bring out the sabre. Other cavalry still used the lance followed up by a sabre.
I am sure some of the kill numbers a few of these cavalrymen racked up running down a beaten foe would definitely qualify as a mass killing.
load outs varied over the eons. I wouldn't be surprised if there weren't times you would have found one armed with a blunderbuss backed up by four pistols.
My history is probably not precise but the point is that mass killings were nothing new at the time the 2nd amendment was written. That the technology of the time certianly enabled a person to kill a lot of unarmed people.